C3 is a complement protein that is activated through cleavage by complement 3 (C3) convertases to C3a and C3b. These are further degraded into iC3b and C3dg/C3d. C3 is crucial in the induction of tolerance generated when an antigen is introduced into immunoprivileged sites and this is exploited by pathogens and cancer cells to evade the immune system by inhibiting complement activation.