Which of the following statements is NOT true of the classical pathway of complement activation?

• A. It requires the presence of C1, C2, and C4 complement proteins.
• B. It is initiated by the binding of antibodies to antigens.
• C. It can be activated in the absence of antibodies.
• D. It leads to the formation of the membrane attack complex (MAC).

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the classical pathway of complement activation. This pathway is one of the three complement activation pathways and is typically initiated by the binding of antibodies (IgG or IgM) to antigens on the surface of a pathogen.
2. Step 2: Identify the key complement proteins involved in the classical pathway. The classical pathway requires the complement proteins C1, C2, and C4 to form the C3 convertase, which is essential for downstream activation.
3. Step 3: Recognize the role of antibodies in the classical pathway. The pathway is antibody-dependent, meaning it requires antibodies bound to antigens to initiate the cascade by activating C1.
4. Step 4: Understand the outcome of the classical pathway activation. Activation leads to a cascade that results in the formation of the membrane attack complex (MAC), which creates pores in the pathogen's membrane, leading to cell lysis.
5. Step 5: Evaluate the statements given in the problem. Since the classical pathway requires antibodies to initiate and involves C1, C2, and C4, the statement 'It can be activated in the absence of antibodies' is NOT true for the classical pathway.