C1 esterase inhibitor's role in the complement system is to:

• A. Inhibits activation of the complement cascade by preventing C1 activation
• B. Cleave C3 to C3b
• C. Activate the classical pathway
• D. Bind to antibodies to initiate the classical pathway

Answer: (A)

C1 esterase inhibitor dampens the classical complement pathway right at its start. It is a serine protease inhibitor that targets C1r and C1s within the C1 complex, preventing C1 from becoming activated after C1q binds to antibody on antigen. By stopping C1 activation, the cascade cannot cleave C4 and C2 to form the C4b2a C3 convertase, so downstream components of the pathway aren’t engaged. It does not cleave C3 to C3b, nor does it activate the classical pathway, nor does it initiate activation by binding antibodies—the initiator is C1q binding to antibody, with C1-INH acting to restrain the process.