Questions: Primary Immunodeficiency Disorders: Classification and Mechanisms

Question 1

A child has recurrent pneumonia and sinusitis caused by Streptococcus pneumoniae and Haemophilus influenzae (both encapsulated bacteria) but recovers normally from viral infections and shows no unusual susceptibility to fungi. Which immune defect best explains this pattern?

• A. T cell deficiency, because T cells are required for clearing all bacterial infections
• B. Complement deficiency, because the membrane attack complex is critical for encapsulated bacteria
• C. Antibody deficiency, because antibodies are the primary mechanism for opsonizing encapsulated bacteria, which evade phagocytosis without opsonization
• D. Phagocyte dysfunction, because neutrophils are the primary defense against extracellular bacteria

Answer: (C)

Encapsulated bacteria like S. pneumoniae and H. influenzae evade phagocytosis by hiding behind their polysaccharide capsule. Antibodies (especially IgG and IgM) bind to the capsule and opsonize the bacteria, flagging them for phagocytes to engulf. Without adequate antibodies, these bacteria are not cleared. Normal viral clearance indicates T cells are intact. Fungal susceptibility would suggest T cell or phagocyte defects. Complement deficiency would especially predispose to Neisseria infections. This pattern is classic for X-linked agammaglobulinemia or other antibody deficiencies.

Question 2

An infant with SCID develops life-threatening Pneumocystis jirovecii pneumonia. Why does the loss of T cells produce such globally devastating susceptibility to opportunistic infections, even beyond the organisms that T cells directly kill?

• A. T cells produce all antibody isotypes, so SCID patients lack all antibody-mediated defenses
• B. T cells are required for most B cell responses, so SCID patients also lack functional antibody production, eliminating both arms of adaptive immunity simultaneously
• C. T cells patrol all mucosal surfaces, and their absence allows organisms to colonize the respiratory and gut epithelia unchecked
• D. T cells produce complement proteins, so SCID patients lack both adaptive and innate humoral defenses

Answer: (B)

This is why SCID is so severe. T helper cells provide the costimulatory signals (CD40L-CD40 interaction, cytokine help) that B cells require to undergo class switching, affinity maturation, and memory formation. Without T cell help, B cells can make only weak IgM responses to T-independent antigens and cannot make protective IgG, IgA, or IgE. So SCID patients effectively lack both cellular immunity (T cells) and humoral immunity (antibodies). This is also why Pneumocystis — a fungal organism normally controlled by combined innate and adaptive defenses — becomes lethal.

Question 3

Chronic granulomatous disease (CGD) causes recurrent infections specifically with catalase-positive organisms like Staphylococcus aureus and Aspergillus because these organisms can neutralize the small amounts of H₂O₂ they produce themselves, leaving CGD neutrophils unable to kill them even after engulfment.

• T. True
• F. False

Answer: True

CGD neutrophils cannot generate the oxidative burst (reactive oxygen species via NADPH oxidase) needed to kill engulfed microbes. Some organisms — catalase-negative bacteria like Streptococcus — produce their own H₂O₂ as a metabolic byproduct, and this H₂O₂ can substitute for the missing oxidative burst, allowing CGD neutrophils to kill them. Catalase-positive organisms (S. aureus, Aspergillus, Burkholderia) produce catalase that breaks down their own H₂O₂, leaving CGD neutrophils without any oxidant to work with. This elegantly explains the organism-specific susceptibility in CGD.

Question 4

Most primary immunodeficiencies present in infancy with severe, life-threatening infections, and any PID that fails to cause symptoms by age 2 should be reclassified as a secondary immunodeficiency.

• T. True
• F. False

Answer: False

This is a significant clinical misconception. Selective IgA deficiency is the most common PID and is frequently asymptomatic — many individuals are identified incidentally on blood tests. Even when symptomatic, IgA deficiency typically causes mild recurrent sinopulmonary infections, not life-threatening disease, because other antibody classes (IgG, IgM) partially compensate. PIDs vary enormously in severity: from lethal SCID presenting in the first months of life to common variable immunodeficiency (CVID) that may not manifest until adulthood. Age of onset and severity depend on which immune component is affected and how much redundancy exists.

Question 5

Why do late complement deficiencies (C5-C9) specifically predispose patients to recurrent Neisseria meningitidis and Neisseria gonorrhoeae infections, while patients with these deficiencies handle most other bacterial infections normally?

Model answer: The terminal complement components C5-C9 form the membrane attack complex (MAC), which inserts into bacterial outer membranes and lyses them. Most bacteria are killed by phagocytosis (opsonization via C3b and antibodies) or by intracellular mechanisms, so they are handled adequately even without MAC. Neisseria species are unusual gram-negative bacteria that are specifically susceptible to MAC-mediated lysis and relatively resistant to intracellular killing after phagocytosis. Without MAC, Neisseria can evade clearance even after being opsonized and engulfed. This is why late complement deficiency produces a narrow susceptibility profile rather than broad immunodeficiency.

This example illustrates the general principle that each arm of immunity specializes against particular pathogen types. Late complement deficiency is also more common in certain populations and can be managed with vaccination (meningococcal vaccine reduces the pathogen burden even without MAC). Understanding the specific function lost predicts the infection vulnerability, which is the clinical diagnostic skill that PID classification teaches.