2.1 Primary & Secondary Immunodeficiencies
Key Takeaways
- X-linked agammaglobulinemia (XLA/Bruton) presents after 6 months of age with <2% CD19+ B cells, pan-hypogammaglobulinemia, absent tonsils/lymph nodes, and recurrent sinopulmonary bacterial infections.
- Common Variable Immunodeficiency (CVID) manifests in young adults (ages 20–40) with reduced IgG/IgA/IgM, normal B cell counts, and elevated risk for autoimmune disease and lymphoma; Selective IgA deficiency is often asymptomatic but carries risk for anaphylactic transfusion reactions due to anti-IgA antibodies.
- Severe Combined Immunodeficiency (SCID) presents in infants (<6 months) with failure to thrive, chronic diarrhea, persistent candidiasis, absolute lymphocyte count <1,500/mm³, and absent thymic shadow on chest radiograph.
- Chronic Granulomatous Disease (CGD) results from defective NADPH oxidase, predisposing patients to recurrent catalase-positive bacterial and fungal infections, and is diagnosed via dihydrorhodamine (DHR) flow cytometry showing absent respiratory burst.
- Secondary immunodeficiencies are far more prevalent than primary immunodeficiencies; AIDS is diagnosed when CD4+ T-cell count falls below 200 cells/mm³ or upon presentation with an AIDS-defining opportunistic illness (e.g., Pneumocystis jirovecii pneumonia, Kaposi sarcoma).
Primary & Secondary Immunodeficiencies
Introduction & Diagnostic Architecture
Immunodeficiencies represent disorders of immune system component failure, divided into primary immunodeficiencies (PIDs)—inherited genetic defects usually presenting in infancy or childhood—and secondary immunodeficiencies, which result from extrinsic factors such as HIV/AIDS, malnutrition, immunosuppressive pharmacotherapy, malignancies (e.g., multiple myeloma, CLL), or post-splenectomy states. On USMLE Step 2 CK, clinical vignettes emphasize matching age of onset, specific pathogen susceptibility patterns, characteristic physical exam findings, and quantitative laboratory markers to narrow the differential diagnosis and initiate appropriate therapy.
Classification of Immunodeficiencies by Immune Arm
| Immune Component | Primary Defect Examples | Susceptible Pathogens | Characteristic Lab Findings |
|---|---|---|---|
| Humoral (B-Cell) | X-linked Agammaglobulinemia, CVID, Selective IgA Deficiency | Encapsulated bacteria (S. pneumoniae, H. influenzae, N. meningitidis), Giardia lamblia | Decreased Ig levels, decreased CD19/CD20+ B cells, absent germinal centers |
| Cellular (T-Cell) | DiGeorge Syndrome, IL-12 Receptor Deficiency | Viruses (CMV, VZV, HSV), fungi (Candida, PCP), mycobacteria | Decreased CD3+ T cells, delayed-type hypersensitivity skin test negativity |
| Combined (B & T) | SCID, Wiskott-Aldrich Syndrome, Ataxia-Telangiectasia | Bacteria, viruses, fungi, protozoa | Low ALC (<1,500/mm³), low immunoglobulins, absent thymic shadow |
| Phagocytic | Chronic Granulomatous Disease, Leukocyte Adhesion Deficiency, Chédiak-Higashi | Catalase-positive organisms (S. aureus, Pseudomonas, Aspergillus, Serratia) | Abnormal DHR assay, persistent neutrophilia without pus formation, giant granules |
| Complement | Early components (C1q, C2, C4), Late components (C5-C9) | Encapsulated bacteria (C1-C4), Neisseria species (C5-C9) | Low CH50 assay, low individual complement component levels |
Primary B-Cell (Humoral) Immunodeficiencies
X-Linked Agammaglobulinemia (Bruton Agammaglobulinemia)
X-linked agammaglobulinemia (XLA) is caused by a loss-of-function mutation in the Bruton tyrosine kinase (BTK) gene, halting pre-B cell maturation into mature B cells in the bone marrow. Symptoms manifest after 6 months of age, as protective maternal IgG antibodies decay. Patients suffer from recurrent sinopulmonary bacterial infections (Streptococcus pneumoniae, non-typeable Haemophilus influenzae) and enteroviral infections (poliovirus, echovirus). Physical examination reveals absent tonsils and lymph nodes. Flow cytometry shows <2% CD19+ mature B cells with markedly decreased levels of all immunoglobulin classes (IgG, IgA, IgM, IgE). Treatment requires lifelong intravenous immunoglobulin (IVIG) or subcutaneous immunoglobulin (SCIG) replacement and prophylactic antibiotics. Live vaccines are strictly contraindicated.
Common Variable Immunodeficiency (CVID)
Common Variable Immunodeficiency (CVID) is the most common symptomatic primary immunodeficiency presenting in late adolescence or early adulthood (ages 20–40). The underlying cause involves heterogeneous defects in B-cell differentiation and plasma cell survival. Patients exhibit recurrent sinopulmonary infections, bronchiectasis, chronic diarrhea (Giardia), and a high incidence of autoimmune diseases (e.g., immune thrombocytopenia, autoimmune hemolytic anemia) and malignancies (especially non-Hodgkin lymphoma and gastric adenocarcinoma). Laboratory findings demonstrate low serum IgG alongside low IgA or IgM, with poor antibody response to vaccination. Management relies on IVIG/SCIG therapy.
Selective IgA Deficiency
Selective IgA Deficiency is the most frequent primary immunodeficiency overall, occurring in approximately 1 in 500 individuals. Most patients are asymptomatic; however, symptomatic individuals present with recurrent mucosal infections (sinusitis, bronchitis) and chronic diarrhea. Crucially, up to 30% of patients develop anti-IgA antibodies. If transfused with blood products containing IgA, these individuals can experience severe, potentially fatal anaphylactic transfusion reactions. Diagnosis is confirmed by serum IgA <7 mg/dL with normal IgG and IgM levels. Treatment involves prompt management of infections; washed red blood cells or IgA-deficient blood products must be utilized for transfusions.
Combined & Cell-Mediated Immunodeficiencies
Severe Combined Immunodeficiency (SCID)
Severe Combined Immunodeficiency (SCID) is a pediatric medical emergency caused by mutations such as X-linked IL-2 receptor gamma chain deficiency (most common) or autosomal recessive adenosine deaminase (ADA) deficiency. Adenosine deaminase deficiency leads to toxic accumulation of dATP, inhibiting ribonucleotide reductase and destroying lymphocytes. Infants present within the first 6 months of life with severe failure to thrive, intractable diarrhea, persistent oral candidiasis, and life-threatening infections caused by bacterial, viral, fungal, and protozoal pathogens (e.g., Pneumocystis jirovecii). Chest radiography demonstrates an absent thymic shadow. Absolute lymphocyte count is typically <1,500/mm³. Definitive treatment is hematopoietic stem cell transplantation (HSCT) before 3.5 months of age to ensure survival.
DiGeorge Syndrome (22q11.2 Deletion)
DiGeorge syndrome stems from microdeletion of chromosome 22q11.2, impairing development of the 3rd and 4th pharyngeal pouches. The classic clinical triad includes:
- Conotruncal cardiac defects (Tetralogy of Fallot, truncus arteriosus)
- Hypoparathyroidism producing hypocalcemia and neonatal tetany
- Thymic aplasia/hypoplasia causing T-cell immunodeficiency
Laboratory evaluation shows decreased CD3+ T cells with normal immunoglobulin levels unless T-cell help is compromised. Management includes correction of hypocalcemia, cardiac repair, and thymic transplantation or HSCT for severe immunodeficiency.
Phagocytic Defects & Secondary Immunodeficiencies
Chronic Granulomatous Disease (CGD)
Chronic Granulomatous Disease (CGD) is predominantly an X-linked recessive disorder caused by mutations in NADPH oxidase, impairing the generation of reactive oxygen species (superoxide radicals) within phagocytes to execute the respiratory burst. Patients are highly susceptible to catalase-positive organisms that degrade endogenous bacterial hydrogen peroxide: Staphylococcus aureus, Pseudomonas cepacia, Serratia marcescens, Aspergillus species, and Nocardia. Recurrent granuloma formation leads to skin abscesses, lymphadenitis, hepatic abscesses, and osteomyelitis. The diagnostic standard is the dihydrorhodamine (DHR) flow cytometry assay (showing lack of fluorescence conversion) or the historic nitroblue tetrazolium (NBT) reduction test. Treatment includes lifelong daily trimethoprim-sulfamethoxazole and itraconazole prophylaxis, along with interferon-gamma injections.
Diagnostic Evaluation Algorithm for Suspected Immunodeficiency
Suspected Recurrent / Severe Infections
├─ Initial Screen: CBC with differential (ALC, ANC), Quantitative immunoglobulins (IgG, IgA, IgM, IgE)
│
├─ Recurrent Sinopulmonary (Humoral Suspicion)
│ └─ Flow Cytometry (CD19/CD20 B cells) + Specific Antibody Response (Tetanus, Pneumococcal)
│ ├─ Decreased IgG/IgA/IgM + <2% B cells → XLA (Bruton)
│ └─ Decreased IgG/IgA/IgM + Normal B cells → CVID
│
├─ Early Infant / Opportunistic Infections (Combined Suspicion)
│ └─ Flow Cytometry (CD3/CD4/CD8 T cells, CD19 B cells, CD56 NK cells) + Chest X-ray
│ ├─ ALC <1,500/mm³ + Absent Thymic Shadow → SCID
│ └─ Decreased CD3+ T cells + Hypocalcemia + Conotruncal Heart Defect → DiGeorge Syndrome
│
└─ Catalase-Positive Infections / Abscesses (Phagocytic Suspicion)
└─ Dihydrorhodamine (DHR) Flow Cytometry Assay
├─ Absent DHR fluorescence → Chronic Granulomatous Disease (CGD)
└─ Neutrophilia without pus at umbilical cord site → Leukocyte Adhesion Deficiency (LAD)
A 10-month-old male infant is brought to the clinic due to his third episode of otitis media and second episode of lobar pneumonia within the past 4 months. Physical examination reveals completely absent tonsils and no palpable peripheral lymph nodes. Laboratory testing demonstrates a total serum IgG level of 120 mg/dL (normal: 500–1200 mg/dL), undetectable serum IgA and IgM levels, and peripheral blood flow cytometry showing <1% CD19+ B cells. Which of the following is the most appropriate long-term management strategy for this patient?
A 26-year-old female presents to the outpatient clinic with a 2-year history of recurrent bacterial sinusitis, chronic diarrhea, and two documented episodes of pneumococcal pneumonia. Routine laboratory studies show a serum IgG level of 210 mg/dL, IgA of 15 mg/dL, and IgM of 22 mg/dL. Flow cytometry reveals normal numbers of circulating CD19+ B lymphocytes, but antibody titers following pneumococcal polysaccharide vaccination show no significant rise. She also reports a history of immune thrombocytopenia diagnosed last year. What is the primary underlying pathophysiology of her condition?
A 4-month-old male infant is evaluated in the hospital for persistent oral candidiasis, failure to thrive, and intractable watery diarrhea. Chest radiograph demonstrates clear lung fields but complete absence of a thymic silhouette. Complete blood count shows a hemoglobin of 11.5 g/dL, platelet count of 250,000/mm³, and white blood cell count of 4,200/mm³ with 85% neutrophils and an absolute lymphocyte count of 480/mm³. Which of the following laboratory findings would most specifically confirm the diagnosis of Severe Combined Immunodeficiency?