Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
Anaphylatoxins, or anaphylotoxins, are fragments (C3a, C4a or C5a) that are produced during the pathways of the complement system.
Direct and indirect functions
Most notable is the ability to trigger degranulation of (release of substances from) mast cells or basophils, which is an important part of the immune system in all kinds of inflammation and especially as part of defense against parasites. If the degranulation is too strong, it can cause allergic reactions.
Anaphylatoxins indirectly mediate:
- spasms of smooth muscle cells, for example bronchospasms
- increase in the permeability of blood capillaries
- chemotaxis — receptor-mediated movement of leukocytes in the direction of the increasing concentration of anaphylatoxins
- C3a is not as strong as C5a, but stronger than C4a and can trigger degranulation of Mast-cells and serve as a chemotactic targeting molecule for eosinophile granulocytes.
- C4a is the least active anaphylatoxin
- C5a is the strongest toxin in the row, it is a very strong mediator of inflammation.
- co-stimulation of C3b at times of phagocytosis (not possible without C5a).
- Chemotactor and activator for granulocytes in general and macrophages
- stimulates respiratory burst in Neutrophils
- increases vascular permeability
- contracts smooth muscle cells
- activates mastcells to churn out Histamine and TNF-Alpha
Although some drugs (morphine, codeine, synthetic ACTH) and some neurotransmitters (norepinephrine, substance P) are important mediators of degranulation of mast cells or basophils, they are generally not called anaphylatoxins. This term is reserved only for fragments of the complement.
- Anaphylatoxin at the US National Library of Medicine Medical Subject Headings (MeSH)