Which of the following statements about complement protein C1 is false?

• A. C1 directly lyses bacterial cells without the need for other complement proteins.
• B. C1 is the first component of the classical complement pathway.
• C. C1 is composed of three subunits: C1q, C1r, and C1s.
• D. C1 activation requires binding to antibodies attached to antigens.

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the role of complement protein C1 in the classical complement pathway. C1 is the initial protein complex that recognizes and binds to antibodies that are attached to antigens on the surface of pathogens.
2. Step 2: Recall the structure of C1. It is composed of three subunits: C1q, which binds to the Fc region of antibodies, and two serine proteases, C1r and C1s, which become activated upon binding.
3. Step 3: Recognize that C1 activation requires the presence of antibodies (IgG or IgM) bound to antigens, which triggers a conformational change leading to activation of C1r and C1s.
4. Step 4: Understand that C1 itself does not directly lyse bacterial cells. Instead, it initiates a cascade involving other complement proteins that ultimately form the membrane attack complex (MAC) responsible for lysis.
5. Step 5: Based on these points, identify the false statement as the one claiming that C1 directly lyses bacterial cells without the need for other complement proteins.