Before We Start
Which infections, not just how many
A patient with recurrent infections isn't enough information on its own — WHICH kinds of infections keep recurring tells you specifically whether the problem lies with B cells, T cells, both together, or a narrow complement gap.
💡 Memory Trick
B cell defects: bacterial infections. T cell defects: viral + fungal + intracellular. Combined: everything. The infection category is the diagnostic signature, not just the frequency of illness.
The Key Points
Four defect categories, four distinct infection signatures
B
B cell / antibody deficiencies — pyogenic bacterial infections
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.
🦠 A child with XLA experiencing recurrent sinopulmonary infections from encapsulated bacteria, while handling most viral illnesses normally, reflects the specific gap left by missing functional antibody production.
T
T cell deficiencies — viral, fungal, and intracellular infections
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) — precisely the categories of pathogen that specifically require T cell-mediated defense to control.
🦠 An infant with DiGeorge syndrome developing recurrent CMV and fungal infections, while handling ordinary bacterial infections without unusual difficulty, reflects a T cell-specific gap rather than a B cell one.
SCID
Combined deficiencies — vulnerability across every category at once
SCID (Severe Combined Immunodeficiency) knocks out both B and T cell function simultaneously, leaving the patient vulnerable to essentially every category of infection — bacterial, viral, fungal, and intracellular all at once, rather than one narrow category.
🦠 An infant with SCID developing severe infections across every category from a very young age reflects the loss of both arms of adaptive immunity together, not just one.
C
Complement deficiency — a narrow, specific exception
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, since it doesn't fit neatly into either category.
🦠 A patient with a specific, recurrent susceptibility to Neisseria infections and nothing else unusual points toward a late complement component (C5-C9) deficiency rather than a general B or T cell problem.
🏥 Applied Scenario
A child has recurrent bacterial sinopulmonary infections (S. pneumoniae, H. influenzae) but handles viral infections normally.
Step 1
Identify the pattern this fits: This pattern points toward a B cell/antibody deficiency, like XLA or CVID, since the infection category is specifically bacterial while viral handling remains normal.
Step 2
Contrast with a different infection pattern: A different infant who instead develops recurrent CMV, fungal, and Mycobacterial infections, but handles ordinary bacterial infections without unusual difficulty, points instead toward a T cell deficiency, like DiGeorge syndrome.
Step 3
Contrast with a broader, combined pattern: A third infant developing severe infections across every category — bacterial, viral, fungal — starting from a very young age points toward a combined deficiency like SCID, rather than either isolated B or T cell defect.
Step 4
Conclusion: The specific category of recurring infection — not just the fact that infections keep recurring — is itself the diagnostic clue that narrows down exactly which part of the immune system is missing.
📌 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 as a narrow exception to the broader pattern.
⚠️ The Trap — Treating All Immunodeficiencies as Causing the Same Broad Vulnerability
The most common trap is treating all immunodeficiencies as if they cause the same broad vulnerability to infection generally. The infection pattern is actually diagnostic and specific: 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 — each pattern narrows the diagnosis meaningfully rather than just confirming "an immune problem exists."
✓ Quick Self-Test
Answer before checking:
1. What infection pattern is characteristic of B cell/antibody deficiency?
2. What infection pattern is characteristic of T cell deficiency?
3. What does SCID affect, and what's the resulting infection pattern?
4. What specific pathogen is associated with late complement component (C5-C9) deficiency?
5. Name two conditions that cause B cell/antibody deficiency.
Answers:
1. Recurrent pyogenic bacterial infections, especially encapsulated organisms like S. pneumoniae and H. influenzae.
2. Viral (CMV, HSV, EBV), fungal (PCP, Candida, Cryptococcus), and intracellular (Toxoplasma, Mycobacteria) infections.
3. Both B and T cell function; patients become vulnerable to essentially every category of infection.
4. Neisseria.
5. XLA (X-linked agammaglobulinemia) and CVID (common variable immunodeficiency).