C3 deficiency vs C5-9 deficiency
How to tell C3 deficiency and C5-9 deficiency apart when a board question makes them look alike.
C3 deficiency is broad severe pyogenic and immune-complex risk; C5-9 deficiency is recurrent Neisseria.
Side by side
C3 deficiency causes severe recurrent encapsulated bacterial infections, immune-complex disease, and impaired opsonization because central complement activation is lost.
C5-9 terminal complement deficiency causes recurrent Neisseria infections because membrane attack complex formation is impaired.
C3 deficiency causes severe recurrent encapsulated bacterial infections, immune-complex disease, and impaired opsonization because central complement activation is lost.
C5-9 terminal complement deficiency causes recurrent Neisseria infections because membrane attack complex formation is impaired.
Severe recurrent encapsulated bacterial infections and immune complex disease; low C3 and low CH50.
Recurrent Neisseria (meningococcal, gonococcal) infections; low CH50 from a missing membrane attack complex.
The repair move
Before naming the answer, force the stem through the separator: C3 deficiency is broad severe pyogenic and immune-complex risk; C5-9 deficiency is recurrent Neisseria.
Try a free question
CoreA 4-year-old boy has had multiple severe pyogenic bacterial infections including Streptococcus pneumoniae bacteremia, Haemophilus influenzae meningitis, and recurrent severe sinopulmonary infections. He also has chronic glomerulonephritis with immune complex deposits. Total complement (CH50) is markedly reduced; specific testing shows undetectable C3. Which condition is most likely?
Show answer and explanation
Answer: A. C3 deficiency. C3 deficiency is severe because C3 is the central convergence point of all three complement pathways (classical, lectin, alternative). C3 generates the opsonin C3b (essential for clearing encapsulated bacteria) and seeds C5 cleavage. Without C3, severe recurrent pyogenic encapsulated bacterial infections (Strep pneumoniae, Hib, Neisseria meningitidis) occur, plus immune complex disease because C3b normally helps clear immune complexes.
Why the other choices are tempting
- B. C5-9 deficiency causes recurrent Neisseria infections (MAC failure), not the broad pyogenic infection plus immune complex pattern of C3 deficiency.
- C. C1-INH deficiency causes hereditary angioedema, not severe recurrent infections.
- D. Asplenia is encapsulated organism risk but does not specifically cause C3 undetectable on CH50.
- E. Sickle cell disease has functional asplenia with encapsulated organism risk but does not cause C3 deficiency.
Flashcards
Tap a card to flip it.
What clue supports C3 deficiency?
Choose C3 deficiency when the stem gives this pattern: C3 deficiency causes severe recurrent encapsulated bacterial infections, immune-complex disease, and impaired opsonization because central complement activation is lost.
What clue supports C5-9 deficiency?
Choose C5-9 deficiency when the stem gives this contrasting pattern: C5-9 terminal complement deficiency causes recurrent Neisseria infections because membrane attack complex formation is impaired.
Quick answers
What is the fastest way to tell C3 deficiency from C5-9 deficiency?
C3 deficiency is broad severe pyogenic and immune-complex risk; C5-9 deficiency is recurrent Neisseria.
What is the key clue for C3 deficiency?
C3 deficiency causes severe recurrent encapsulated bacterial infections, immune-complex disease, and impaired opsonization because central complement activation is lost.
What is the key clue for C5-9 deficiency?
C5-9 terminal complement deficiency causes recurrent Neisseria infections because membrane attack complex formation is impaired.