Question

A case of recurrent Neisseria gonorrhoeae infection is being evaluated. Which immunological investigation would best detect a complement pathway defect?

• A. C1 esterase inhibitor assay
• B. Quantitative immunoglobulin levels
• C. Nitroblue tetrazolium test
• D. Terminal complement (C5--C9) assay

Hint:

Recurrent Neisseria infections always point toward terminal complement deficiency, especially C5--C9 deficiency.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the specific immune system defect that predisposes a patient to recurrent infections with Neisseria species (Gonorrhea or Meningitis).
Step 2: Detailed Explanation:

Role of Complement: The complement system is a crucial part of innate immunity. The "terminal" components (C5, C6, C7, C8, and C9) assemble to form the Membrane Attack Complex (MAC).

Membrane Attack Complex (MAC): The MAC's primary function is to punch holes in the cell membranes of Gram-negative bacteria, leading to osmotic lysis.

Neisseria Susceptibility: Neisseria species (N. gonorrhoeae and N. meningitidis) have thin cell walls and are uniquely sensitive to lysis by the MAC. Individuals with deficiencies in any of the terminal complement components (C5-C9) cannot form the MAC and therefore suffer from recurrent, often disseminated, Neisserial infections.

Why not others?
- C1 esterase inhibitor (A): Deficiency causes Hereditary Angioedema, not recurrent infections.
- Quantitative Immunoglobulins (B): Deficiency (e.g., CVID) leads to recurrent infections with encapsulated bacteria (S. pneumoniae, H. influenzae).
- NBT test (C): Used to diagnose Chronic Granulomatous Disease (defective oxidative burst in neutrophils).

Step 3: Final Answer:
A terminal complement (C5-C9) assay is indicated to identify a MAC defect in patients with recurrent Neisserial disease.