The formation of the C3 convertase is a key step in complement activation that occurs in all three complement pathways. This enzyme cleaves C3 in blood plasma, leading to a conformational change in the C3b fragment that exposes its reactive thioester group. The activated C3b is potentially harmful to the host, if it becomes covalently attached to a host cell, rather than to the surface of a pathogen. This deleterious outcome is largely avoided by:

Correct Answer: C

Rationale: C3b hydrolyzes quickly in solution , preventing host damage; it can diffuse , attach to host cells , or bind convertase but is regulated.

Correct Answer: B

Rationale: The inflammatory response is primarily triggered by tissue injury, not specifically microbial infection or erythrocyte migration, though infection can lead to injury.

Correct Answer: C

Rationale: Septic shock is a systemic inflammatory response to bacterial infection, potentially fatal, unlike anaphylaxis (allergic), pelvic disease (local), or pneumonia (lung-specific).

Correct Answer: C

Rationale: Interleukin-1 from antigen-presenting cells (e.g., macrophages) activates helper T cells, unlike interferon, complement, or perforin (cytotoxic T cell product).

Correct Answer: D

Rationale: The classical pathway, triggered by antibodies, promotes inflammation, lysis (via MAC), and opsonization (via C3b), making all options correct.