The complement system is a major humoral factor for tagging and killing target microorganisms. A striking feature of bony fish complement is the diversity of several complement components. The present review is focused on the diversity of C3, C4, and C5, which belong to the α2-macroglobulin (α2M) family. Except for C5, they contain an internal thioester bond, which mediates covalent binding of the proteins to target cells and molecules. In contrast to mammals, carp has multiple C3 isoforms, including novel isoforms that lack the catalytic histidine residue that primarily determines the reactivity of the thioester to amino- and hydroxyl-groups and thereby influences the binding specificity of the components. Protein analyses revealed that the C3 isoforms differ in the hemolytic activity and the binding specificity. Carp also have two diverged C4 isotypes, designated C4A and C4B. C4B has the catalytic histidine and C4A does not, like human C4B and C4A isotypes respectively, predicting their difference in the binding specificity. C5 is also present in two distinct forms in carp. One of them contains a potentially deleterious amino acid substituion. These diversity in the complement components may represent a unique strategy to enhance innate immunity of bony fish.
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