Allophycocyanin is an accessory pigment to bacteriochlorophyll and is part of the phycobilin accessory pigment family, along with phycocyanin and phycoerythrin. All phycobilins, short for phycobiliproteins, are water soluble and therefore cannot exist within the membrane like carotenoids, but aggregate forming clusters that adhere to the membrane called phycobilisomes. Allophycocyanin absorbs red light, therefore emits blue, and is readily found in blue-green algae, now classified as Cyanobacteria, and red algae. Phycobilin pigments have fluorescent properties that are used in immunoassay kits. Allophycocyanin is from the Greek allo meaning "other," phyco meaning “algae” and cyanin is from the English “cyan.” In flow cytometry, it is often abbreviated to APC. An accessory pigment is a pigment other than chlorophyll found in plants, such as a carotenoid, that serves the function of absorbing light energy, and transfering it to chlorophyll. ... Bacteriochlorophylls are photosynthetic pigments that occur in various bacteria. ... Phycobilins are the chromophores of phycobiliproteins (photosynthetic pigments) found in cyanobacteria and in the chloroplasts of red algae, glaucophytes and some cryptomonads (though not in green algae and higher plants). ... Phycocyanin is a pigment that is blue and is readily found in blue-green algae. ... Phycoerythrin is a red protein from the light-harvesting phycobiliproteins family, isolated from red, blue-green, and crytomonad algae. ... Carotenoids are organic pigments naturally occurring in plants and some other photosynthetic organisms like algae, some types of fungus and some bacteria. ... A computer generated 3D view of a phycobilisome showing Phycoerythrin subunits in red, Phycocyanin subunits in dark blue and Allophycocyanin subunits in light blue. ... Cyanobacteria (Greek: cyanos = blue) are a phylum of aquatic bacteria that obtain their energy through photosynthesis. ... Orders The taxonomy of the Cyanobacteria is currently under revision. ... An immunoassay is a biochemical test that measures the level of a substance in a biological liquid, typically serum or urine, using the reaction of an antibody or antibodies to its antigen. ... Cyan is a pure spectral color, but the same hue can also be generated by mixing equal amounts of green and blue light. ... Flow cytometry is a technique for counting, examining and sorting microscopic particles suspended in a stream of fluid. ... APC is an abbreviation of: General A Perfect Circle, rock band Advanced process control Air Pollution Control in municipal solid waste incineration plants Angled Physical Contact Fiber Optic Connector Antipop Consortium, an alternative hip-hop group Armoured personnel carrier Armour-piercing capped shot and shell Automatic Passenger Counter Automatic Performance...