The Classic Pathway of Complement Activation Begins When the Protein

Question

The classic pathway of complement activation begins when the protein C1 binds to
A) the cell wall of bacteria.
B) the plasma membrane of bacteria.
C) two antibodies attached to an antigen.
D) a cell surface antigen.
E) a plasma protein.

Accepted Answer

The classic pathway of complement activation is initiated by the binding of C1 (a complex of three complement proteins: C1q, C1r, and C1s) to two antibodies (IgG or IgM) that are bound to an antigen. This triggers a cascade of complement proteins that ultimately results in the formation of the membrane attack complex (MAC) and lysis of the target cell.