Antigen presentation stimulates T cells to become either "cytotoxic" CD8+ cells or "helper" CD4+ cells.

T helper cells (also known as effector T cells or T_h cells) are a sub-group of lymphocytes (a type of white blood cell or leukocyte) that plays an important role in establishing and maximizing the capabilities of the immune system. These cells are unusual in that they have no cytotoxic or phagocytic activity; they cannot kill infected host (also known as somatic) cells or pathogens, and without other immune cells they would usually be considered useless against an infection. T_h cells are involved in activating and directing other immune cells, and are particularly important in the immune system. They are essential in determining B cell antibody class switching, in the activation and growth of cytotoxic T cells, and in maximizing bactericidal activity of phagocytes such as macrophages. It is this diversity in function and their role in influencing other cells that gives T helper cells their name. Image File history File links Antigen_presentation. ... Image File history File links Antigen_presentation. ... A lymphocyte is a type of white blood cell involved in the human bodys immune system. ... “White Blood Cells” redirects here. ... White Blood Cells is also the name of a White Stripes album. ... A scanning electron microscope image of a single neutrophil (yellow), engulfing anthrax bacteria (orange). ... Cytotoxicity is the quality of being poisonous to cells. ... Phagocytosis (literally cell-eating) is a form of endocytosis wherein large particles are enveloped by the cell membrane of a (usually larger) cell and internalized to form a phagosome, or food vacuole. ... The term somatic refers to the body, as distinct from some other entity, such as the mind. ... A pathogen (literally birth of pain from the Greek παθογένεια) is a biological agent that can cause disease to its host. ... A scanning electron microscope image of a single neutrophil (yellow), engulfing anthrax bacteria (orange). ... B cells are lymphocytes that play a large role in the humoral immune response (as opposed to the cell-mediated immune response). ... Mechanism of class switch recombination that allows isotype switching in activated B cells Immunoglobulin class switching (or isotype switching or Isotypic Commutation) is a biological mechanism that changes an antibody from one class to another, for example, from an isotype called IgM to an isotype called IgG. During this process... A cytotoxic (or TC) T cell is a T cell (a type of white blood cell) which has on its surface antigen receptors that can bind to fragments of antigens displayed by the Class I MHC molecules of virus (or other intracellular pathogen) infected somatic cells and tumor cells. ... A bacteriocide or bactericide is a substance that kills bacteria and, preferably, nothing else. ... A phagocyte is a cell that ingests (and destroys) foreign matter, such as microorganisms or debris via a process known as phagocytosis, in which these cells ingest and kill offending cells by cellular digestion. ... Macrophages (Greek: big eaters) are cells found in tissues that are responsible for phagocytosis of pathogens, dead cells and cellular debris. ...

Mature T_h cells are believed to always express the surface protein CD4. T cells expressing CD4 are also known as CD4⁺ T cells. CD4⁺ T cells are generally treated as having a pre-defined role as helper T cells within the immune system, although there are known rare exceptions. For example, there are sub-groups of suppressor T cells, natural killer T cells, and cytotoxic T cells that are known to express CD4 (although cytotoxic examples have been observed in extremely low numbers in specific disease states, they are usually considered non-existent). All of the latter CD4⁺ T cell groups are not considered T helper cells, and are beyond the scope of this article. CD4 (cluster of differentiation 4) is a molecule that is expressed on the surface of T helper cells (as well as regulatory T cells and dendritic cells). ... A scanning electron microscope image of a single neutrophil (yellow), engulfing anthrax bacteria (orange). ... This article or section should be merged with regulatory T cell There were debates over the existence of suppressor cells, but recent years studies have confirmed a central role of suppressor cell populations in regulating immunity. ... Natural killer T cells (NK T cells) are a type of lymphocyte, or white blood cell. ... A cytotoxic (or TC) T cell is a T cell (a type of white blood cell) which has on its surface antigen receptors that can bind to fragments of antigens displayed by the Class I MHC molecules of virus (or other intracellular pathogen) infected somatic cells and tumor cells. ...

The importance of helper T cells can be seen from HIV, a virus that infects cells that are CD4⁺ (including helper T cells). Towards the end of a HIV infection the number of functional CD4⁺ T cells falls, which leads to the symptomatic stage of infection known as the acquired immune deficiency syndrome (AIDS). There are also rare disorders, probably genetic in etiology, that result in the absence or dysfunction of CD4⁺ T cells. These disorders produce similar symptoms, and many of these are fatal (see T-Lymphocytopenia). Species Human immunodeficiency virus 1 Human immunodeficiency virus 2 Human immunodeficiency virus (HIV) is a retrovirus that causes acquired immunodeficiency syndrome (AIDS, a condition in humans in which the immune system begins to fail, leading to life-threatening opportunistic infections). ... For other uses, see AIDS (disambiguation). ... T-Lymphocytopenia (ICL) is a depletion of CD4+ lymphocytes below 300 per cubic millimeter in the absence of HIV infection or other known causes of immunodeficiency. ...

Activation of naïve helper T cells

Following T cell development, matured, naïve (meaning they have never been exposed to the antigen to which they can respond) T cells leave the thymus and begin to spread throughout the body, including the lymph nodes. Like all T cells, they express the T cell receptor/CD3 complex. The T cell receptor (TcR) consists of both constant and variable regions, the latter of which determines what antigen the T cell can respond to. CD4⁺ T cells have TcRs with an affinity for Class II MHC, and it is believed that CD4 is involved in determining MHC affinity during maturation in the thymus. Class II MHC proteins are generally only found on the surface of professional antigen-presenting cells (APCs). Professional antigen presenting cells are primarily dendritic cells, macrophages and B cells, although dendritic cells are the only cell group that expresses MHC Class II constitutively (at all times). Some APCs also bind native (or unprocessed) antigens to their surface, such as follicular dendritic cells, but unprocessed antigens do not interact with T cells and are not involved in their activation. The antigens that bind to MHC proteins are always short peptides, 8-10 amino acids long for MHC Class I, and up to 25 or so for MHC Class II. T cells are a subset of lymphocytes that play a large role in the immune response. ... For the server security software, see Microsoft Forefront. ... Thymus, see Thyme. ... Lymph nodes are components of the lymphatic system. ... Antigen presentation stimulates T cells to become either cytotoxic CD8+ cells or helper CD4+ cells. ... In immunology, the CD3 antigen (CD stands for cluster of differentiation) is a protein complex composed of three distinct chains (CD3γ, CD3δ and CD3ε) in mammals, that associate with molecules known as the T cell receptor (TCR) and the ζ-chain to generate an activation signal in T lymphocytes. ... Thymus, see Thyme. ... The major histocompatibility complex (MHC) is a large genomic region or gene family found in most vertebrates containing many genes with important immune system roles. ... An antigen presenting cell (APC) is a cell that displays foreign antigen complexed with MHC on its surface. ... Dendritic cells (DC) are immune cells and form part of the mammal immune system. ... Macrophages (Greek: big eaters) are cells found in tissues that are responsible for phagocytosis of pathogens, dead cells and cellular debris. ... B cells are lymphocytes that play a large role in the humoral immune response (as opposed to the cell-mediated immune response). ... Dendritic cells (DC) are immune cells and form part of the mammal immune system. ... Peptides are the family of molecules formed from the linking, in a defined order, of various amino acids. ...

Recognition (Signal 1)

During an immune response, professional antigen-presenting cells (APCs) endocytose (absorb) foreign material (typically bacteria or viruses), which undergoes processing, then travel from the site of infection to the lymph nodes. Once at the lymph nodes, the APC begins to present antigen peptides that are bound to Class II MHC, allowing CD4⁺ T cells that express specific TcR's against the peptide/MHC complex to activate. An antigen-presenting cell (APC) is a cell that displays foreign antigen complexed with MHC on its surface. ... It has been suggested that Endocytic cycle be merged into this article or section. ... Phyla Actinobacteria Aquificae Chlamydiae Bacteroidetes/Chlorobi Chloroflexi Chrysiogenetes Cyanobacteria Deferribacteres Deinococcus-Thermus Dictyoglomi Fibrobacteres/Acidobacteria Firmicutes Fusobacteria Gemmatimonadetes Lentisphaerae Nitrospirae Planctomycetes Proteobacteria Spirochaetes Thermodesulfobacteria Thermomicrobia Thermotogae Verrucomicrobia Bacteria (singular: bacterium) are unicellular microorganisms. ... This article is about biological infectious particles. ... Two methods exist for an antigen to be processed and presented on the cell surface. ... Lymph nodes are components of the lymphatic system. ...

When a T_h cell encounters and recognises the antigen on an APC, the TcR-CD3 complex binds strongly to the peptide-MHC complex present on the surface of professional APC's. CD4, a co-receptor of the TCR complex, also binds to a different section of the MHC molecule. These interactions brings these proteins closer together, allowing the intracellular kinases present on the TcR, CD3 and CD4 proteins to activate each other via phosphorylation. With the assistance of another phosphatase present on the intracellular section of CD45 (common leukocyte antigen), these molecules activate the major biochemical pathways in the cytosol of the T_h cell. These active pathways are known as Signal 1 of T cell activation, as it is the first and primary pro-activation signal in a T_h cell. Upon subsequent encounters with a given antigen, memory T cells are re-activated using the same TCR pathways. Antigen presentation stimulates T cells to become either cytotoxic CD8+ cells or helper CD4+ cells. ... CD3 or CD-3 is an abbreviation with multiple meanings, including: Ford CD3 platform MediaMax CD-3, a copy protection scheme a 3-inch CD or MiniCD This is a disambiguation page — a navigational aid which lists pages that might otherwise share the same title. ... CD4 (cluster of differentiation 4) is a molecule that is expressed on the surface of T helper cells (as well as regulatory T cells and dendritic cells). ... In cell biology, molecular biology and related fields, the word intracellular means inside the cell. It is used in contrast to extracellular (outside the cell). ... In biochemistry, a kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific target molecules (substrates); the process is termed phosphorylation. ... A phosphorylated serine residue Phosphorylation is the addition of a phosphate (PO4) group to a protein or a small molecule or the introduction of a phosphate group into an organic molecule. ... In immunology, the CD45 antigen (CD stands for cluster of differentiation) is a protein which is a leucocyte common antigen, a type I transmembrane protein present on all differentiated hematopoietic cells except erythrocytes that assists in the activation of those cells (a form of co-stimulation). ... The cytosol (cf. ... A lymphocyte is a shown in the center of this picture Memory T cells are a specific type of infection-fighting T cell (also known as a T lymphocyte) that can recognize foreign invaders such as bacteria or viruses, that were encountered during a prior infection or vaccination. ...

The binding of the antigen-MHC to the TCR complex and CD4 may also help the APC and the T_h cell adhere during T_h cell activation, but the protein LFA-1 on the T cell and ICAM on the APC are the primary molecules of adhesion in this cell interaction. LFA-1 stands for Lymphocyte function-associated antigen-1. ... Intercellular adhesion molecules (ICAMs) are molecules that promote adhesion between cells. ...

It is unknown what role the relatively bulky extracellular region of CD45 plays during cell interactions, but CD45 has various isoforms that change in size depending on the T_h cell's activation and maturation status. For example, CD45 shortens in length following T_h activation (CD45RA⁺ to CD45RO⁺), but whether this change in length influences activation is unknown. It has been proposed that the larger CD45RA+ may decrease the accessibility of the T cell receptor for the antigen-MHC molecule, thereby necessitating an increase in the affinity (and specificity) of the T cell for activation. Once the activation has occurred however, CD45 shortens, allowing easier interactions and activation as an effector T helper cell.^{[citation needed]}

Verification (Signal 2)

Having received the first TcR/CD3 signal, the naïve T cell must activate a second independent biochemical pathway, known as Signal 2. This verification step is a protective measure to ensure that a T cell is responding to a foreign antigen. If this second signal is not present during initial antigen exposure, the T cell presumes that it is auto-reactive. This results in the cell becoming anergic (anergy is generated from the unprotected biochemical changes of Signal 1). Anergic cells will not respond to any antigen in the future, even if both signals are present later on. These cells are generally believed to circulate throughout the body with no value until they apoptose at the end of their lifespan. Anergy is a theory in immunobiology in which there is a lack of reaction by the bodys defence mechanisms when foreign substances come into contact with the body. ... A section of mouse liver showing an apoptotic cell indicated by an arrow Apoptosis (pronounced apo tō sis) is a process of suicide by a cell in a multicellular organism. ...

The second signal involves an interaction between CD28 on the CD4⁺ T cell and the proteins CD80 (B7.1) or CD86 (B7.2) on the professional APCs. Both CD80 and CD86 activate the CD28 receptor. These proteins are also known as co-stimulatory molecules. Structure of human CD28. ... The protein CD80 is a molecule which provides a costimulatory signal necessary for T cell activation and survival. ... The protein CD86 is a molecule which provides a costimulatory signal necessary for T cell activation and survival. ... During the activation of T cells, co-stimulation of molecules is often crucial to the development of an effective immune response. ...

Although the verification stage is necessary for the activation of naïve helper T cells, the importance of this stage is best demonstrated during the similar activation mechanism of CD8⁺ cytotoxic T cells. As naïve CD8⁺ T cells have no true bias towards foreign sources, these T cells must rely on the activation of CD28 for confirmation that they recognise a foreign antigen (as CD80/CD86 is only expressed by active APC's). CD28 plays an important role in decreasing the risk of T cell auto-immunity against host antigens. A cytotoxic T cell (also known as TC, CTL or killer T cell) belongs to a sub-group of T lymphocytes (a type of white blood cell) which are capable of inducing the death of infected somatic or tumor cells; they kill cells that are infected with viruses (or other...

Once the naïve T cell has both pathways activated, the biochemical changes induced by Signal 1 are altered, allowing the cell to activate instead of anergise. The second signal is then obsolete; only the first signal is necessary for future activation. This is also true for memory T cells, which is one example of learned immunity. Faster responses occur upon reinfection because memory T cells have already undergone confirmation and can produce effector cells much sooner. Categories: Possible copyright violations ...

Proliferation

Once both stimulatory signals are active within the helper T cell, the cell then allows itself to proliferate. It achieves this by releasing a potent T cell growth factor called interleukin-2 (IL-2). Activated T cells also produce the alpha sub-unit of the IL-2 receptor (CD25 or IL-2R), enabling a fully functional receptor that can bind with IL-2, which in turn activates the T cell's proliferation pathways. The term cell growth is used in two different ways in biology. ... Interleukin-2 (IL2) is an interleukin, a type of biological response modifier that can improve the bodys natural response to disease. ... The interleukin-2 receptor (IL-2R) is heterotrimeric protein expressed on the surface of certain immune cells, such as lymphocytes, that binds and responds to a cytokine called interleukin 2. ...

In this case, the released IL-2 binds to same T cell's IL-2 receptors to allow itself to proliferate. The phenomenon of cells releasing cytokines to alter their own behaviour is known as auto-regulation (or autocrine stimulation). It should be noted that this is not the only function of IL-2 release, and that IL-2 can also bind to other T cells in the area (paracrine stimulation). Autocrine signaling is a form of signalling in which a cell secretes a chemical messenger (called the autocrine agent) that signals the same cell. ...

Maturation

After many cell generations, the T_h cell's progenitors differentiate into effector T_h cells, memory T_h cells, and suppressor T_h cells.

• Effector T_h cells secrete cytokines, proteins or peptides that stimulate or interact with other leukocytes, including T_h cells.

• Memory T_h cells retain the antigen affinity of the originally activated T cell, and are used to act as later effector cells during a second immune response (e.g. if there is re-infection of the host at a later stage).

• Suppressor T cells do not promote immune function, but act to decrease it instead. Despite their low numbers during an infection, these cells are believed to play an important role in the self-limitation of the immune system; they have been shown to prevent the development of various auto-immune diseases.

The production of IL-2 by helper T cells is also necessary for the proliferation of activated CD8⁺ T cells. Without helper T cell interactions, CD8⁺ T cells do not proliferate and eventually become anergic. This cross-reliance on helper T cells is another way the immune system tries to prevent T cell-mediated auto-immune disease. Cytokines are a group of proteins and peptides that are used in organisms as signaling compounds. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... Peptides (from the Greek πεπτος, digestible), are the family of short molecules formed from the linking, in a defined order, of various α-amino acids. ... White Blood Cells is also the name of a White Stripes album. ... This article or section should be merged with regulatory T cell There were debates over the existence of suppressor cells, but recent years studies have confirmed a central role of suppressor cell populations in regulating immunity. ... Autoimmune diseases arise from an overactive immune response of the body against substances and tissues normally present in the body. ... A cytotoxic T cell (also known as TC, CTL or killer T cell) belongs to a sub-group of T lymphocytes (a type of white blood cell) which are capable of inducing the death of infected somatic or tumor cells; they kill cells that are infected with viruses (or other... CD8 (cluster of differentiation 8) is a molecule that is expressed on the surface of cytotoxic T cells. ... Anergy is a theory in immunobiology in which there is a lack of reaction by the bodys defence mechanisms when foreign substances come into contact with the body. ... Autoimmune diseases arise from an overactive immune response of the body against substances and tissues normally present in the body. ...

Determination of the effector T cell response

Helper T cells are capable of influencing a variety of immune cells, and the T cell response generated (including the extracellular signals such as cytokines) can be essential for a successful outcome from infection. In order to be effective, helper T cells must determine which cytokines will allow the immune system to be most useful or beneficial for the host. Understanding exactly how helper T cells respond to immune challenges is currently of major interest in immunology, because such knowledge may be very useful in the treatment of disease and in increasing the effectiveness of vaccination. In cell biology, molecular biology and related fields, the word extracellular means outside the cell. It is used in contrast to intracellular (inside the cell). ... Cytokines are a group of proteins and peptides that are used in organisms as signaling compounds. ... Immunology is a broad branch of biomedical science that covers the study of all aspects of the immune system in all organisms. ... This article is about the medical term. ... A vial of the vaccine against influenza. ...

T_h1/T_h2 Model for helper T cells

Proliferating helper T cells that develop into effector T cells differentiate into two major subtypes of cells known as T_h1 and T_h2 cells (also known as Type 1 and Type 2 helper T cells, respectively).

While we know about the types of cytokine patterns helper T cells tend to produce, we understand less about how the patterns themselves are decided. Various evidence suggests that the type of APC presenting the antigen to the T cell has a major influence on its profile. Other evidence suggests that the concentration of antigen presented to the T cell during primary activation influences its choice. The presence of some cytokines (such as the ones mentioned above) will also influence the response that will eventually be generated, but our understanding is nowhere near complete. Interferon-gamma or IFN-g is a dimerized soluble cytokine which is a Type II Interferon. ... Lymphotoxin (also known as tumor necrosis factor-beta) is a lymphokine cytokine. ... Interleukin-2 (IL2) is an interleukin, a type of biological response modifier that can improve the bodys natural response to disease. ... Interleukin-4, abreviated IL-4, is a lymphokine that stimulates the proliferation of activated B-cells and T-cells, among other effects. ... IL-5 is an interleukin produced by T helper-2 cells and mast cells. ... Interleukin-6 (IL-6) is a pro-inflammatory cytokine secreted by T cells and macrophages to stimulate immune response to trauma, especially burns or other tissue damage leading to inflammation. ... Interleukin-10 (IL-10 or IL10), also known as human cytokine synthesis inhibitory factor (CSIF), is an anti-inflammatory cytokine, capable of inhibiting synthesis of pro-inflammatory cytokines like IFN-gamma, IL-2, IL-3, TNFα and GM-CSF by cells such as macrophages and Th1 cells. ... Interleukin 13 (IL-13) is a cytokine secreted by many cell types, but especially T helper type 2 (Th2) cells, that is an important mediator of allergic inflammation and disease. ... Cell-mediated immunity is an immune response that does not involve antibodies but rather involves the activation of macrophages and NK-cells, the production of antigen-specific cytotoxic T-lymphocytes, and the release of various cytokines in response to an antigen. ... Macrophages (Greek: big eaters) are cells found in tissues that are responsible for phagocytosis of pathogens, dead cells and cellular debris. ... CD8 (cluster of differentiation 8) is a molecule that is expressed on the surface of cytotoxic T cells. ... Humoral immunity is mediated by secreted antibodies, produced in cells of the B lymphocyte lineage (B cell). ... B cells are lymphocytes that play a large role in the humoral immune response (as opposed to the cell-mediated immune response). ... Mechanism of class switch recombination that allows isotype switching in activated B cells Immunoglobulin class switching (or isotype switching or Isotypic Commutation) is a biological mechanism that changes an antibody from one class to another, for example, from an isotype called IgM to an isotype called IgG. During this process... Each antibody binds to a specific antigen; an interaction similar to a lock and key. ... Interferon-gamma or IFN-g is a dimerized soluble cytokine which is a Type II Interferon. ... Interleukin 12 (IL-12) is an interleukin that are naturally produced by macrophages and human B-lymphoblastoid cells (NC-37)in response to antigenic stimulation. ... Interleukin-4, abreviated IL-4, is a lymphokine that stimulates the proliferation of activated B-cells and T-cells, among other effects. ... Interleukin-4, abreviated IL-4, is a lymphokine that stimulates the proliferation of activated B-cells and T-cells, among other effects. ... Interleukin-10 (IL-10 or IL10), also known as human cytokine synthesis inhibitory factor (CSIF), is an anti-inflammatory cytokine, capable of inhibiting synthesis of pro-inflammatory cytokines like IFN-gamma, IL-2, IL-3, TNFα and GM-CSF by cells such as macrophages and Th1 cells. ... Interleukin-2 (IL2) is an interleukin, a type of biological response modifier that can improve the bodys natural response to disease. ... Interferon-gamma or IFN-g is a dimerized soluble cytokine which is a Type II Interferon. ...

Limitations to the T_h1/T_h2 model

The interactions between cytokines from the T_h1/T_h2 model can be more complicated in some animals. For example, the T_h2 cytokine IL-10 inhibits cytokine production of both T_h subsets in humans. Human IL-10 (hIL-10) suppresses the proliferation and cytokine production of all T cells and the activity of macrophages, but continues to stimulate plasma cells, ensuring that antibody production still occurs. As such, hIL-10 is not believed to truly promote the T_h2 response in humans, but acts to prevent over-stimulation of helper T cells while still maximising the production of antibodies. Interleukin-10 (IL-10 or IL10), also known as human cytokine synthesis inhibitory factor (CSIF), is an anti-inflammatory cytokine, capable of inhibiting synthesis of pro-inflammatory cytokines like IFN-gamma, IL-2, IL-3, TNFα and GM-CSF by cells such as macrophages and Th1 cells. ... Plasma cells are B lymphocytes that secrete immunoglobulins (antibodies). ... Each antibody binds to a specific antigen; an interaction similar to a lock and key. ...

There are also other types of T cells that can influence the expression and activation of helper T cells, such as natural regulatory T cells, along with less common cytokine profiles such as the T_h3 subset of helper T cells. Terms such as "regulatory" and "suppression" have become ambiguous after the discovery that helper CD4⁺ T cells are also capable of regulating (and suppressing) their own responses outside of dedicated suppressor T cells. This article or section should be merged with suppressor T cell Regulatory T cells (also known as suppressor T cells) are characterised by virtue of their expression of the cell surface markers CD4 and CD25. ...

One major difference with "suppressor" (or "natural regulatory") T cells is that they always suppress the immune system, while effector T cell groups usually begin with immune-promoting cytokines and then switch to inhibitory cytokines later in their repertoire. The latter is a feature of T_h3 cells, which transform into a suppressor subset after its initial activation and cytokine production. Immunosuppression is the medical suppression of the immune system. ...

Both regulatory T cells and T_h3 cells produce the cytokine transforming growth factor-beta (TGF-β) and IL-10. Both cytokines are inhibitory to helper T cells; TGF-β suppresses the activity of most of the immune system. There is evidence to suggest that TGF-β may not suppress activated Th2 cells as effectively as it might suppress naive cells, but it is not typically considered a Th2 cytokine. This article or section should be merged with suppressor T cell Regulatory T cells (also known as suppressor T cells) are characterised by virtue of their expression of the cell surface markers CD4 and CD25. ... Transforming Growth Factor beta (TGF beta) is a biological protein. ...

The characterisation of another novel T helper subtype, T helper 17 cells (T_h17)^[1] has cast further doubt on the basic T_h1/T_h2 model. These [interleukin 17|IL-17]] producing cells were initially described as a pathogenic population implicated in autoimmunity but are now thought to have their own distinct effector and regulatory functions. T helper 17 cells (Th17) are a subset of interleukin 17-producing T helper cells. ...

Many of the cytokines in this article are also expressed by other immune cells (see individual cytokines for details), and it is becoming clear that while the original T_h1/T_h2 model is enlightening and gives insight into the functions of helper T cells, it is far too simple to define its entire role or actions. Some immunologists question the model completely, as some in vivo studies suggest that individual helper T cells usually do not match the specific cytokine profiles of the T_h model, and many cells express cytokines from both profiles. That said, the T_h model has still played an important part in developing our understanding of the roles and behaviour of helper T cells and the cytokines they produce during an immune response.

Role of helper T cells in disease

Considering the diverse and important role helper T cells play in the immune system, it is not surprising that these cells often influence the immune response against disease. They also appear to make occasional mistakes, or generate responses that would be politely considered non-beneficial. In the worst case scenario, the helper T cell response could lead to a disaster and the fatality of the host. Fortunately this is a very rare occurrence.

Helper T cells and hypersensitivity

The immune system must achieve a balance of sensitivity in order to respond to foreign antigens without responding to the antigens of the host itself. When the immune system responds to very low levels of antigen that it usually shouldn't respond to, a hypersensitivity response occurs. Hypersensitivity is believed to be the cause of allergy and some auto-immune disease. Hypersensitivity refers to undesirable (damaging, discomfort-producing and sometimes fatal) reactions produced by the normal immune system. ... An allergy is an abnormal, acquired sensitivity to a given substance, including pollen, drugs, or numerous environmental triggers. ... Autoimmune diseases arise from an overactive immune response of the body against substances and tissues normally present in the body. ...

Hypersensitivity reactions can be divided into four types:

Type 1 hypersensitivity includes common immune disorders such as asthma, allergic rhinitis (hay fever), eczema, urticaria (hives) and anaphylaxis. These reactions all involve IgE antibodies, which require a T_h2 response during helper T cell development. Preventative treatments, such as corticosteroids and montelukast, focus on suppressing mast cells or other allergic cells; T cells do not play a primary role during the actual inflammatory response. It's important to note that the numeral allocation of hypersensitivity "types" does not correlate (and is completely unrelated) to the "response" in the T_h model. Allergy is an abnormal reaction to a substance foreign to the body that is acquired, predictable and rapid. ... For the play, see Hay Fever. ... For the beetle, see Exema. ... Anaphylaxis is an acute systemic (multi-system) and severe Type I Hypersensitivity allergic reaction in humans and other mammals. ... IGE (Internet Gaming Entertainment) is the largest MMORPG services company world-wide, with offices in Los Angeles, Hong Kong, and Miami. ... Each antibody binds to a specific antigen; an interaction similar to a lock and key. ... In physiology, corticosteroids are a class of steroid hormones that are produced in the adrenal cortex. ... Montelukast is a leukotriene receptor antagonist (LTRA) used for the maintenance treatment of asthma and to relieve symptoms of seasonal allergies. ...

Type 2 and Type 3 hypersensitivity both involve complications from auto-immune or low affinity antibodies. In both of these reactions, T cells may play an accomplice role in generating these auto-specific antibodies, although some of these reactions under Type 2 hypersensitivity would be considered normal in a healthy immune system (for example, Rhesus factor reactions during child-birth is a normal immune response against child antigens). The understanding of the role of helper T cells in these responses is limited but it is generally thought that T_h2 cytokines would promote such disorders. For example, studies have suggested that lupus (SLE) and other auto-immune diseases of similar nature can be linked to the production of T_h2 cytokines. A blood type is a description an individuals characteristics of red blood cells due to substances (carbohydrates and proteins) on the cell membrane. ... Systemic lupus erythematosus (SLE or lupus) is a chronic autoimmune disease that can be fatal, though with recent medical advances this is becoming increasingly rare. ...

Type 4 hypersensitivity, also known as delayed type hypersensitivity, are caused via the over-stimulation of immune cells, commonly lymphocytes and macrophages, resulting in chronic inflammation and cytokine release. Antibodies do not play a direct role in this allergy type. T cells play an important role in this hypersensitivity, as they activate against the stimulus itself and promote the activation of other cells; particularly macrophages via T_h1 cytokines. Hypersensitivity refers to undesirable (damaging, discomfort-producing and sometimes fatal) reactions produced by the normal immune system. ... A lymphocyte is a type of white blood cell involved in the human bodys immune system. ... Macrophages (Greek: big eaters) are cells found in tissues that are responsible for phagocytosis of pathogens, dead cells and cellular debris. ... An abscess on the skin, showing the redness and swelling characteristic of inflammation. ...

Other cellular hypersensitivities include cytotoxic T cell mediated auto-immune disease, and a similar phenomenon; transplant rejection. Helper T cells are required to fuel the development of these diseases. In order to create sufficient auto-reactive killer T cells, interleukin-2 must be produced, and this is supplied by CD4⁺ T cells. CD4⁺ T cells can also stimulate cells such as natural killer cells and macrophages via cytokines such as interferon-gamma, encouraging these cytotoxic cells to kill host cells in certain circumstances. A cytotoxic T cell (also known as TC, CTL or killer T cell) belongs to a sub-group of T lymphocytes (a type of white blood cell) which are capable of inducing the death of infected somatic or tumor cells; they kill cells that are infected with viruses (or other... Autoimmune diseases arise from an overactive immune response of the body against substances and tissues normally present in the body. ... Transplant rejection occurs when the immune system of the recipient of a transplant attacks the transplanted organ or tissue. ... Autoimmunity is the failure of an organism to recognize its own constituent parts (down to the sub-molecular levels) as self, which results in an immune response against its own cells and tissues. ... Interleukin-2 (IL2) is an interleukin, a type of biological response modifier that can improve the bodys natural response to disease. ... Natural NK cells are cytotoxic; small granules in their cytoplasm contain special proteins such as perforin and proteases known as granzymes. ... Interferon-gamma or IFN-g is a dimerized soluble cytokine which is a Type II Interferon. ...

The mechanism that killer T cells use during auto-immunity is almost identical to their response against viruses, and some viruses have been accused of causing auto-immune diseases such as Type 1 Diabetes mellitus. Cellular auto-immune disease occurs because the host antigen recognition systems fail, and the immune system believes, by mistake, that a host antigen is foreign. As a result, the CD8⁺ T cells treat the host cell presenting that antigen as infected, and go on to destroy all host cells (or in the case of transplant rejection, transplant organ) that express that antigen. Stop editing pages god ... For the disease characterized by excretion of large amounts of very dilute urine, see diabetes insipidus. ... A scanning electron microscope image of a single neutrophil (yellow), engulfing anthrax bacteria (orange). ...

It should be noted that some of this section is a simplification, and that many auto-immune diseases are more complex; a well known example is rheumatoid arthritis, where both antibodies and immune cells are known to play a role in the pathology. Generally the immunology of most auto-immune diseases is not well understood. Rheumatoid arthritis (RA) is traditionally considered a chronic, inflammatory autoimmune disorder that causes the immune system to attack the joints. ...

HIV infection

Perhaps the best example of the importance of CD4⁺ T cells is demonstrated with human immunodeficiency virus (HIV) infection. HIV targets cells that express CD4, and can infect macrophages, dendritic cells (both groups express CD4 at low levels) and CD4⁺ T cells. CD4 (cluster of differentiation 4) is a molecule that is expressed on the surface of T helper cells (as well as regulatory T cells and dendritic cells). ... Species Human immunodeficiency virus 1 Human immunodeficiency virus 2 Human immunodeficiency virus (HIV) is a retrovirus that causes acquired immunodeficiency syndrome (AIDS, a condition in humans in which the immune system begins to fail, leading to life-threatening opportunistic infections). ... Macrophages (Greek: big eaters) are cells found in tissues that are responsible for phagocytosis of pathogens, dead cells and cellular debris. ... Dendritic cells (DC) are immune cells and form part of the mammal immune system. ...

It has been proposed that during the non-symptomatic phase of HIV infection, the virus has a relatively low affinity towards T cells (and has a higher affinity for macrophages), resulting in a slow kill rate of CD4⁺ T cells by the immune system. This is initially compensated for via the production of new helper T cells from the thymus (originally from the bone marrow). Once the virus becomes lymphotropic (or T-tropic) however, it begins to infect CD4⁺ T cells far more efficiently (likely due to a change in the co-receptors it binds to during infection), and the immune system is overwhelmed. Thymus, see Thyme. ... This article does not cite any references or sources. ... Species Human immunodeficiency virus 1 Human immunodeficiency virus 2 Human immunodeficiency virus (HIV) is a retrovirus that causes acquired immunodeficiency syndrome (AIDS, a condition in humans in which the immune system begins to fail, leading to life-threatening opportunistic infections). ... This page is a candidate to be copied to Wiktionary. ...

At this point, functional CD4⁺ T cell levels begin to decrease, eventually to a point where the CD4⁺ T cell population is too small to recognize the full range of antigens that could potentially be detected. The lack of full antigen cover results in the core symptoms of acquired immune deficiency syndrome (AIDS). CD4 T cell deleption during AIDS allows various pathogens to escape T cell recognition, thus allowing opportunistic infections that would normally elicit a helper T cell response to bypass the immune system. While these complete bypass situations only occur when the helper T cell response is absolutely necessary for infection clearance, most infections increase in severity and/or duration because the immune system's helper T cells provide a weaker contribution to a less efficient immune response. For the server security software, see Microsoft Forefront. ... For other uses, see AIDS (disambiguation). ... A pathogen (literally birth of pain from the Greek παθογένεια) is a biological agent that can cause disease to its host. ... Opportunistic infections are infections in immunodeficient patients caused by pathogens which are incapable of causing infection in immunocompetent individuals. ...

Two components of the immune system are particularly affected in AIDS, due to its CD4⁺ T cell dependency:

1. CD8⁺ T cells are not stimulated as effectively during the AIDS period of HIV infection, making AIDS patients very susceptible to most viruses, including HIV itself. This decline in killing of CD4⁺ T cells results in the virus being produced for a longer period (the infected CD4⁺ T cells are not killed as quickly), increasing the proliferation of the virus, and accelerating the development of the disease.
2. Antibody class switching declines significantly once helper T cell function fails. The immune system loses its ability to improve the affinity of their antibodies, and are unable to generate B cells that can produce antibody groups such as IgG and IgA. These effects are primarily due to the loss of any helper T cell that can interact with the B lymphocyte correctly. Another symptom of AIDS is the reduction in antibody levels due to a decrease in T_h2 cytokines (and less interactions by helper T cells). All of these complications result in an increased susceptibility to aggressive bacterial infections, especially in areas of the body not accessible by IgM antibodies.

If the patient does not respond to (or does not receive) HIV treatment they will succumb usually to either cancers or infections; the immune system finally reaches a point where it is no longer coordinated or stimulated enough to deal with the disease. CD8 (cluster of differentiation 8) is a molecule that is expressed on the surface of cytotoxic T cells. ... Mechanism of class switch recombination that allows isotype switching in activated B cells Immunoglobulin class switching (or isotype switching or Isotypic Commutation) is a biological mechanism that changes an antibody from one class to another, for example, from an isotype called IgM to an isotype called IgG. During this process... Schematic of antibody binding to an antigen An antibody is a protein complex used by the immune system to identify and neutralize foreign objects like bacteria and viruses. ... IGA may stand for: Koji Igarashi, a video game producer Interactive genetic algorithm International Geothermal Association Independent Glass Association International Gothic Association International Gamers Award International Goat Association Irish Games Association Irish Geological Association ImmunoGlobulin A - see IgA nephritis which is a renal disease IGA (supermarkets) Independent Grocers Association or... IGM might be an acronym or abbreviation for: The polymeric immunoglobulin, IgM International Grandmaster, a chess ranking intergalactic medium Intragroup medium - see: Intracluster medium IG Metall - the dominant German metalworkers union IGM is an acronym created by Robinson Technologies for several early BBS door games, including Legend of the Red... Species Human immunodeficiency virus 1 Human immunodeficiency virus 2 Human immunodeficiency virus (HIV) is a retrovirus that causes acquired immunodeficiency syndrome (AIDS, a condition in humans in which the immune system begins to fail, leading to life-threatening opportunistic infections). ...

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