**Question:** C3 convertase acts on -

A. C3b inactivation
B. C4b2a complex
C. C5 convertase
D. C3a desensitization

**Core Concept:** C3 convertase is a crucial enzyme in the complement system, a part of the immune response. The complement system helps to identify and destroy pathogens, modulate inflammation, and clear immune complexes from the body.

**Why the Correct Answer is Right:** C3 convertase is formed by the cleavage of C4b2a complex (B) into C4b2a3 complex. This complex activates C3b and generates C3a and C4a, which are anaphylatoxins involved in inflammation and immune response. C3 convertase then cleaves C3 into C3a and C3b, which contributes to the amplification loop of the complement system.

**Why Each Wrong Option is Incorrect:**

A. C3b inactivation (C3b is a key component of the complement pathway, not a target of C3 convertase).
C. C5 convertase (C5 convertase is responsible for the final cleavage of C5, not C3 convertase).
D. C3a desensitization (This option is incorrect because C3a is an anaphylatoxin, not a target of C3 convertase).

**Clinical Pearl:** The complement system plays a vital role in host defense, and its dysregulation can lead to various immune-mediated diseases, including autoimmune disorders, atypical hemolytic uremic syndrome (aHUS), and age-related macular degeneration (AMD).

**Correct Answer:** C3 convertase acts on C3b, which is generated from C3 during the activation process. C3b plays a crucial role in the amplification loop and opsonization of pathogens, marking them for destruction by phagocytic cells.