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Difficulty: HardImmunologic and Inflammatory Processes

A 24-year-old woman comes to the clinic for evaluation of a widespread, erythematous rash on her cheeks and the sun-exposed areas of her arms. She also reports progressive joint pain and swelling in her wrists and fingers over the past 3 months. Her medical history is significant for several episodes of severe pneumococcal pneumonia and bacterial sinusitis during childhood and adolescence. Her temperature is 37.6°C (99.7°F), blood pressure is 138/88 mm Hg, and pulse is 88/min. Physical examination reveals a prominent malar rash and synovitis of the metacarpophalangeal joints bilaterally. Laboratory studies show:

TestResultReference Range
Antinuclear antibodiesPositive (1:640)Negative
Anti-double-stranded DNAPositiveNegative
Total hemolytic complement (CH50)< 10 U/mL41–90 U/mL
Alternative pathway hemolytic activity (AH50)85%> 70%

Which of the following is the most likely primary immunopathologic consequence of this patient's underlying quantitative serum defect?

  1. Impaired reticuloendothelial clearance of circulating immune complexes and apoptotic bodiesAnswer
  2. B
    Inability to form transmembrane pores in the lipid bilayer of gram-negative diplococci
  3. C
    Failure to assemble the C3bBb convertase enzyme on microbial cell surfaces
  4. D
    Unregulated activation of the kinin system leading to excessive bradykinin production
  5. E
    Defective assembly of the phagocyte NADPH oxidase complex during the respiratory burst

Answer

Impaired reticuloendothelial clearance of circulating immune complexes and apoptotic bodies
The patient's clinical presentation (early-onset SLE-like syndrome and recurrent childhood pyogenic infections) combined with an undetectable CH50 and normal AH50 indicates a primary deficiency of an early classical complement pathway component (C1q, C2, or C4). The early classical complement pathway is physiologically essential for the clearance of circulating immune complexes and apoptotic debris. During normal function, C1q binds to antibodies within immune complexes, leading to the activation of C2 and C4. This sequence covalently deposits C3b onto the immune complexes. Erythrocytes, which bear complement receptor 1 (CR1), bind these C3b-tagged complexes and transport them to the reticuloendothelial system (primarily the spleen and liver) where they are safely stripped and cleared by macrophages. A deficiency in C1, C2, or C4 halts this process, resulting in the prolonged circulation, accumulation, and eventual tissue deposition of these immune complexes. This leads directly to Type III hypersensitivity reactions manifesting as lupus nephritis, arthritis, and cutaneous lupus.

Step-by-Step Solution

1
Analyze the laboratory profile of the complement system functional assays.
The patient has an undetectable CH50 (measuring the classical and terminal pathways) but a normal AH50 (measuring the alternative and terminal pathways).
Differentiating between classical, alternative, and terminal complement pathway defects requires analyzing which pathways are non-functional. A normal AH50 rules out a terminal pathway (C5-C9) defect.
2
Synthesize the lab findings to identify the precise pathway defect.
The combination of low CH50 and normal AH50 isolates the defect exclusively to the early classical complement pathway (C1, C2, or C4).
Since the terminal pathway is intact (evidenced by normal AH50), the CH50 failure must be due to an early classical component missing.
3
Correlate the classical pathway defect with the patient's clinical presentation.
The patient has a primary deficiency of an early classical complement component (most commonly C2), which manifests clinically as an early-onset systemic lupus erythematosus (SLE)-like syndrome and susceptibility to encapsulated bacteria.
The early classical pathway is physiologically essential for tagging immune complexes and apoptotic cellular debris for removal by the reticuloendothelial system. Without it, immune complexes deposit in tissues, causing Type III hypersensitivity.

Key Concept

Early classical complement pathway deficiencies (C1, C2, C4) are characterized by an undetectable CH50 with a normal AH50, and they classically predispose patients to severe systemic lupus erythematosus due to impaired clearance of immune complexes.
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Topics

Immunologic and Inflammatory Processes

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