Step by Step
B
B cell / antibody deficiencies
Conditions like XLA (X-linked agammaglobulinemia) and CVID (common variable immunodeficiency) impair antibody production, leading to recurrent pyogenic bacterial infections — especially encapsulated organisms like S. pneumoniae and H. influenzae — along with Giardia and enteroviral encephalitis.
T
T cell deficiencies
Conditions like DiGeorge syndrome (thymic aplasia) and HIV impair T cell function, leading to susceptibility to viral infections (CMV, HSV, EBV), fungal infections (PCP, Candida, Cryptococcus), and intracellular pathogens (Toxoplasma, Mycobacteria) — the categories of pathogen that specifically require T cell-mediated defense.
SCID
Combined deficiencies
SCID (Severe Combined Immunodeficiency) knocks out both B and T cell function, leaving the patient vulnerable to essentially every category of infection — bacterial, viral, fungal, and intracellular all at once.
C
Complement deficiency — a special case
Deficiency in the terminal complement components (C5-C9), which form the membrane attack complex, causes a very specific vulnerability to Neisseria infections in particular — a useful, narrowly-testable exception to the broader B cell/T cell pattern.
Applied Walkthrough
1
A child has recurrent bacterial sinopulmonary infections (S. pneumoniae, H. influenzae) but handles viral infections normally. This pattern points toward a B cell/antibody deficiency, like XLA or CVID.
2
A different infant develops recurrent CMV, fungal, and Mycobacterial infections, but handles ordinary bacterial infections without unusual difficulty. This pattern instead points toward a T cell deficiency, like DiGeorge syndrome.
3
A third infant develops severe infections across every category — bacterial, viral, fungal — starting from a very young age. This broader vulnerability pattern points toward a combined deficiency like SCID.
4
A fourth patient has a specific, recurrent susceptibility to Neisseria infections and nothing else unusual — this narrow, specific pattern points toward a late complement component (C5-C9) deficiency rather than a general B or T cell problem.
Exam Application
Exams test whether you can match a described infection pattern (bacterial-only, viral/fungal/intracellular, or combined) to the correct type of immunodeficiency, and whether you know the specific Neisseria association with late complement component deficiency.
⚠ Common Trap
The most common trap is treating all immunodeficiencies as if they cause the same broad vulnerability. The infection pattern is diagnostic: bacterial-only points to B cells, viral/fungal/intracellular points to T cells, everything-at-once points to combined deficiency, and Neisseria-specific points to late complement deficiency.
✓ Quick Self-Check
1. What infection pattern is characteristic of B cell/antibody deficiency?
Recurrent pyogenic bacterial infections, especially encapsulated organisms like S. pneumoniae and H. influenzae.
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2. What infection pattern is characteristic of T cell deficiency?
Viral (CMV, HSV, EBV), fungal (PCP, Candida, Cryptococcus), and intracellular (Toxoplasma, Mycobacteria) infections.
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3. What does SCID affect, and what's the resulting infection pattern?
Both B and T cell function; patients become vulnerable to essentially every category of infection.
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4. What specific pathogen is associated with late complement component (C5-C9) deficiency?
Neisseria.
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5. Name two conditions that cause B cell/antibody deficiency.
XLA (X-linked agammaglobulinemia) and CVID (common variable immunodeficiency).
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