A lymphocyte is a type of white blood cell involved in the human body's immune system. There are two broad categories of lymphocytes, namely T cells and B cells. Lymphocytes play an important and integral part of the body's defenses.

T-cells are responsible for cell mediated immunity whereas B-cells are responsible for humoral immunity (relating to antibodies). T-cells are named such because these lymphocytes mature in the thymus and B-cells mature in bone marrow.

In the presence of an antigen, B-cells become much more metabolically active and transform into Plasma cells. Plasma cells are large lymphocytes with a large nuclear to cytoplasmic ratio and are the form of B-cell lymphocytes that produce antibodies. Azurophilic granules are often present with an increase in cytoplasm.

Microscopically, in a Wright's stained peripheral blood smear, a normal lymphocyte has a large, dark-staining nucleus with little to no basophyllic cytoplasm. In normal situations, the coarse, dense nucleus of a lymphocyte is approximately the same size as a red blood cell (about 7 micrometres in diameter). Some lymphocytes show a clear perinuclear zone (or halo) around the nucleus or could exhibit a small clear zone to one side of the nucleus.

It is impossible to distinguish between T-cells and B-cells in a peripheral blood smear. Normally, flow cytometry testing is used for specific lymphocyte population counts. When one must specifically determine the percentage of lymphocytes that produce a particular secretion (say, a specific antibody or cytokine), the ELISPOT technique can be used instead.

The Human Immunodeficiency Virus hijacks and destroys T-cell lymphocytes (specifically, CD4 lymphocytes). Without this key defense, the body is susceptible to opportunistic diseases that otherwise would not kill healthy people.

A lymphocyte count is part of a peripheral complete blood cell count and is expressed as percentage of lymphocytes to total white blood cells counted. An increase in lymphocytes is usually a sign of a viral infection (rarely, leukemias are found through an abnormally raised lymphocyte count in an otherwise normal person). A general increase in the number of lymphocytes is known as lymphocytosis whereas a decrease is lymphocytopenia.