## **Core Concept**
Primary immune deficiencies are disorders in which part of the body's immune system is missing or functions improperly. Most primary immune deficiencies are genetic disorders, resulting from defects in the genes that control the development and function of the immune system. These deficiencies can affect various components of the immune system, including B cells, T cells, and complement proteins.

## **Why the Correct Answer is Right**
The correct answer, **C. Complement**, is related to primary immune deficiencies that affect the complement system. The complement system is a part of the innate immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promoting inflammation and enhancing the ability of the immune system to eliminate pathogens. Some primary immune deficiencies, such as Complement Deficiency Diseases, involve deficiencies in one or more of the complement proteins (C1 to C9), which can predispose individuals to infections and autoimmune diseases.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not directly relate to a commonly recognized plasma protein fraction associated with primary immune deficiencies.
- **Option B:** This option is incorrect as it does not specify a known plasma protein fraction related to immune deficiency.
- **Option D:** This option is incorrect because, similar to options A and B, it does not directly correspond to a well-defined plasma protein fraction associated with primary immune deficiencies.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that deficiencies in the complement system, particularly in the terminal complement components (C5-C9), increase susceptibility to Neisseria infections, such as recurrent Neisseria meningitidis infections. This association is critical for clinicians to consider in patients with recurrent infections.

## **Correct Answer:** . Complement