C3 convease acts on

**Core Concept**
Complement component 3 (C3) is a central protein in the complement system, a part of the innate immune response. The complement system helps to eliminate pathogens from the body by marking them for destruction and facilitating their clearance. C3 convertase is a key enzyme that plays a crucial role in the activation of the complement cascade.

**Why the Correct Answer is Right**
C3 convertase acts on C3, converting it into C3a and C3b. This activation step is crucial for the amplification of the complement response. C3 convertase is formed by the interaction of C4b and 2a, which are produced during the classical and alternative pathways of the complement cascade, respectively. The binding of C3b to the pathogen surface marks it for destruction and facilitates its clearance by phagocytic cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because C3 convertase does not act on antibodies. While antibodies are involved in the activation of the classical complement pathway, C3 convertase acts on the C3 protein itself.
**Option B:** This option is incorrect because C3 convertase does not act on the complement component C5. While C5 is an important component of the complement system, C3 convertase is specific for C3.
**Option C:** This option is incorrect because C3 convertase does not act on the complement component C1. While C1 is involved in the activation of the classical complement pathway, C3 convertase acts on C3.

**Clinical Pearl / High-Yield Fact**
The complement system plays a crucial role in the defense against bacterial and viral infections. Deficiencies in the complement system, such as C3 deficiency, can lead to increased susceptibility to infections.

**Correct Answer:** D.