All of the following are true of the recognition unit in the classical pathway except

• A. It consists of C1q, C1r, and C1s.
• B. The subunits require calcium for binding together.
• C. Binding occurs at the FC region of antibody molecules.
• D. C1q becomes an active esterase.

Answer: (D)

The recognition unit of the classical pathway is the C1 complex, made up of C1q bound to two C1r and two C1s molecules. C1q is the part that detects the immune complex by binding to the Fc region of antibodies (IgG or IgM) that are attached to antigen. This binding is Ca2+-dependent, and calcium helps hold the subunits together so the complex can activate properly.

Once C1q binds, C1r is autoactivated and then activates C1s. Active C1s then cleaves the downstream components C4 and C2 to propagate the cascade. Importantly, C1q itself does not gain enzymatic (esterase) activity; the proteolytic activity resides in C1r and C1s after activation. Therefore, saying that C1q becomes an active esterase is not true.