Figure 1.

ABO incompatible destruction of RBCs by the classical pathway of complement. In this example, transfused cells expressing A antigen have been exposed to incompatible recipient serum containing anti-A IgM. The bound IgM molecule is then recognized by serum complement C1 complex. Upon binding to IgM, C1 activates the classical pathway, resulting in complement deposition on the donor cell and eventual clearance or lysis of host RBCs by the membrane attack complex (MAC). For clarity, only C1 binding and MAC formation are illustrated here.