**Question:** Which complement component is anaphylotoxin?

**Core Concept:** Complement system is a part of the immune system that helps to destroy pathogens by forming a membrane attack complex (MAC) on the surface of bacteria and viruses. Anaphylotoxins are a group of proteins that act as potent mediators of inflammation and connective tissue damage.

**Why the Correct Answer is Right:** C5a is a crucial component of the complement system, which is activated through the classical, lectin, or alternative pathway. C5a is an anaphylatoxin, meaning it stimulates mast cells, basophils, and neutrophils leading to release of inflammatory mediators. This results in increased vascular permeability, chemotaxis, and degranulation, which contributes to the inflammatory response.

**Why Each Wrong Option is Incorrect:**

A. C3a is also an anaphylatoxin, but it is produced earlier in the complement cascade, before C5 is converted into C5a and C5b. C3a causes smooth muscle contraction, vasodilation, and increased vascular permeability, but its effects are less potent than C5a.

B. C4a is also produced earlier in the complement cascade, before C5 is converted into C5a and C5b. C4a causes smooth muscle contraction, vasodilation, and increased vascular permeability, but its effects are less potent than C5a.

C. C6a is not an anaphylatoxin, as it is not a product of the complement cascade. C6a is a peptide generated during the formation of the membrane attack complex (MAC) and is not directly responsible for mediating inflammation.

D. C7a is not an anaphylatoxin, as it is not a product of the complement cascade. C7a is a peptide generated during the formation of the membrane attack complex (MAC) and is not directly responsible for mediating inflammation.

**Clinical Pearl:** Understanding complement system activation and anaphylatoxins helps to appreciate the role of these molecules in immune response and inflammation. Knowledge of these components can aid in diagnosing complement deficiencies and understanding their clinical implications, such as increased susceptibility to infections and autoimmune diseases.