C4A is part of a “complement” group. The term complement means it is able to kill bacteria and contributes to immune defenses. However, if there are too many compliments, it can cause tissue damage and trigger an allergic reaction. C4A is an activation protein, which means it also activates the other complement proteins to increase in level. The C3a, C4a, and C5a components are referred to as anaphylatoxins: they cause smooth muscle contraction, histamine release from mast cells, and enhanced vessel permeability. They also mediate inflammation and the generation of free ...
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