**Question:** All of the following are associated with low complement levels, EXCEPT:

A. Deficiency of factor D
B. Deficiency of factor B
C. Deficiency of C3
D. Deficiency of factor H

**Core Concept:** Complement system is part of the immune system that plays a crucial role in the clearance of pathogens, immune complex removal, and regulation of inflammation. Complement proteins work together in a cascade system to produce an effector molecule called C3b, which binds to pathogens and immune complexes, facilitating their clearance by phagocytes. Complement system components include factors A, B, C3, and H, among others.

**Why the Correct Answer is Right:** Factor H is crucial for the regulation of the complement system, particularly by inhibiting the alternative pathway. When factor H levels are low, it leads to increased activation of the alternative pathway, leading to a hyperactive complement system and increased inflammation. The other options, however, are directly involved in the production of effector molecules (factor D) or part of the cascade (factors B and C3).

**Why Each Wrong Option is Incorrect:**

A. Deficiency of factor D: Factor D is involved in the activation of the alternative pathway of the complement system, converting C3bBb to C3bBbP. When factor D is deficient, there is impaired activation of the alternative pathway, resulting in decreased complement activation and a reduced inflammation response.

B. Deficiency of factor B: Factor B is a key component of the alternative pathway, where it forms a complex with factor D to produce C3 convertase (C3bBb). Deficiency of factor B leads to decreased activation of the alternative pathway, resulting in reduced complement activation and inflammation.

C. Deficiency of C3: C3 is an essential component of the complement system, where it is the central intermediate in the cascade and a substrate for cleavage by various enzymes, including C3 convertase. Deficiency of C3 results in impaired complement activation and decreased inflammation.

D. Deficiency of factor H: Factor H is involved in the negative regulation of the alternative pathway, inhibiting the formation of C3 convertase and preventing excessive complement activation. Low levels of factor H lead to enhanced complement activation and inflammation.

**Clinical Pearl:**

Understanding the role of complement system components in regulating inflammation is crucial for interpreting potential clinical presentations in patients with complement deficiencies. A hyperactive complement system (as in factor H deficiency) can lead to increased inflammation and susceptibility to infections, while a hypoactive complement system (as in factor D, B, and C3 deficiencies) can result in decreased inflammation, increased susceptibility to infections, and impaired clearance of immune complexes.