C3, complement component 3, is a 195 KD glycoprotein heterodimer that is linked by disulfide bonds. It is the fourth complement component to react in the classical pathway and it is also a reactant in the alternative complement pathway. C3 convertase splits C3 into C3b and C3a. C3b can bind covalently, via its reactive thioester, to cell surface carbohydrates or immune aggregates. Macrophages and neutrophils have receptors for C3b. C3a is released into the surrounding fluids. It can bind to receptors on basophils and mast cells triggering them to release their vasoactive contents (e.g., histamine). Because of the role of these materials in anaphylaxis, C3a is called an anaphylatoxin. In blood, the C3a fragment is rapidly desarginated by serum carboxypeptidase N enzyme to generate C3a desArg. Both C3a and C3a desArg are not glycosylated.