C3 is the central complement amplifier — its deficiency causes broad opsonization failure and susceptibility to encapsulated bacteria like Strep pneumoniae, H. influenzae, and Neisseria. But recurrent Neisseria infections specifically point to MAC deficiency (C5–C9), not C3. The reason is structural: gram-negative bacteria like Neisseria have an outer membrane that shields them from phagocytosis, and only the membrane attack complex can physically punch through it. Opsonization alone is not enough.
The common mistake
On a complement quiz about a patient with recurrent Neisseria meningitidis infections, Omar picked C3 — the central amplification hub. The reasoning is understandable: C3 is the most important complement component, it's at the convergence of all three pathways, and Neisseria does technically require complement for clearance. C3 feels like the right answer for any complement deficiency question.
A lot of students anchor on C3 as the default "complement deficiency" answer. It is the most commonly tested complement component, and it produces the most dramatic susceptibility. The problem is that not all complement deficiencies produce the same susceptibility pattern — and Neisseria specifically requires the MAC.
The actual mechanism
Complement does two distinct jobs: opsonization and lysis.
C3b, generated from C3 cleavage, coats bacteria and tags them for phagocytosis. This is opsonization. Lose C3, lose opsonization → broad susceptibility to encapsulated bacteria (Strep pneumo, H. flu, Neisseria, Klebsiella). This overlaps with the asplenic patient pattern for the same reason: both depend on opsonization followed by phagocytosis.
The membrane attack complex (MAC) is assembled from C5b through C9. It forms a literal pore in a bacterial outer membrane and lyses the cell directly. The MAC is specific to gram-negative organisms because they have the outer membrane it inserts into. Gram-positive bacteria lack an outer membrane entirely — their thick peptidoglycan wall is the outermost layer, and it's too dense for the MAC to penetrate.
Neisseria is gram-negative. It has an outer membrane. Phagocytosis of Neisseria requires complement opsonization, but killing Neisseria — especially once it's in the bloodstream — depends on MAC-mediated lysis. Patients who lack terminal complement components (C5–C9) can opsonize Neisseria fine but cannot lyse it. The result is recurrent Neisseria bacteremia and meningitis.
The diagnostic pattern:
- C3 deficiency → recurrent encapsulated bacteria across multiple species (Strep pneumo, H. flu, Neisseria, Klebsiella) — broad susceptibility
- MAC deficiency (C5–C9) → recurrent Neisseria specifically — narrow susceptibility
The Neisseria-only pattern is the Step 1 hook. When the vignette mentions repeated Neisseria meningitidis or gonorrhea infections and asks for the complement deficiency, the answer is terminal complement (C5–C9), not C3. Complement deficiency is one pattern of immune dysfunction; for a contrasting pattern — where the defect is T cell-based rather than complement-based — the Candida host-pattern distinction applies the same "match the defect to the function" logic.
How to remember it
C3 = tag (opsonize). MAC = punch (lyse). Neisseria needs to be punched, not just tagged. Any gram-negative bacteria that evades phagocytosis needs MAC.
Alternatively: C3 deficiency looks like asplenia (same bugs, same mechanism). MAC deficiency has a Neisseria-only calling card.
Check yourself
A 22-year-old college student has had two episodes of N. meningitidis bacteremia in three years. Complement levels are ordered. Which finding would you expect?
A) Low C3, normal C5–C9
B) Normal C3, low C5–C9
C) Low C1q, normal C3 and C5–C9
D) Low C4, normal C3 and C5–C9
Answer: B. Recurrent Neisseria specifically points to MAC deficiency — loss of terminal complement components C5–C9. C3 deficiency produces a broader susceptibility pattern across multiple encapsulated bacteria, not a Neisseria-selective pattern.
Close the gap
The tutor that redirected Omar from C3 to C5–C9 — and then confirmed the distinction held on cold recall the next session — is the same system available for your preparation right now.