Primary Immune Deficiency Diseases¶
Chapter 362 | Part 11: Immune-Mediated, Inflammatory, and Rheumatologic Disorders
KEY CLINICAL POINTS¶
- Primary immune deficiencies (PIDs) are genetic disorders with Mendelian inheritance, affecting innate, adaptive, or regulatory pathways.
- Classification of PIDs includes deficiencies of innate immune system (e.g., SCID, CGD), adaptive immune system (e.g., T-cell, B-cell defects), and regulatory defects (e.g., ALPS, IPEX).
- Diagnostic workup includes clinical history, physical exam, and laboratory tests (e.g., serum Ig levels, CH50/AP50, TREC analysis).
- Treatment options include immunoglobulin replacement, HSCT, gene therapy, and targeted therapies (e.g., IFN- γ for MSMD).
- Prognosis varies widely, with severe PIDs requiring early intervention to prevent life-threatening infections and complications.
1. DEFINITION & OVERVIEW¶
Primary immune deficiencies (PIDs) are genetic disorders characterized by intrinsic defects in the immune system, leading to increased susceptibility to infections, autoimmunity, or malignancies. They are classified based on the affected immune component (innate, adaptive, or regulatory) and the underlying molecular defect.
Table 362-1: Classification of Primary Immune Deficiency Diseases¶
| Category | Examples | Key Features |
|---|---|---|
| Innate Immune Deficiencies | SCID, CGD, LAD | Defects in phagocytosis, complement, or TLR pathways |
| Adaptive Immune Deficiencies | T-cell defects (e.g., WAS), B-cell defects (e.g., XLA) | Impaired antibody production, T-cell function |
| Regulatory Defects | ALPS, IPEX, MSMD | Autoimmunity, lymphoproliferation, cytokine dysregulation |
1.1 Genetic Basis¶
PIDs are primarily Mendelian disorders, with over 550 conditions described. Mutations in ~500 genes are implicated, with ~50% of cases involving autosomal recessive inheritance and ~30% X-linked recessive.
1.2 Classification¶
PIDs are categorized into three main groups: (1) Innate immune deficiencies (e.g., SCID, CGD), (2) Adaptive immune deficiencies (e.g., T-cell, B-cell defects), and (3) Regulatory defects (e.g., ALPS, IPEX).
2. EPIDEMIOLOGY¶
PIDs have an estimated prevalence of 5–10 per 100,000 individuals. SCID occurs in ~1 in 50,000 live births, while other PIDs vary in frequency. Genetic counseling is critical due to the hereditary nature of most PIDs.
2.1 Risk Factors¶
Family history of PIDs, consanguinity, and certain genetic syndromes (e.g., DiGeorge syndrome) increase risk. Age-related immune decline (immunosenescence) also contributes to susceptibility.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
PIDs arise from mutations in genes encoding proteins critical for immune cell development, signaling, or function. Defects in cytokine signaling (e.g., IL-2, IL-7), DNA repair (e.g., AT, NBS), or complement pathways (e.g., C3, C5) are common mechanisms.
Table 361-5: Monoclonal Antibodies for Autoimmune Diseases¶
| Target | Function | Approved mAbs | Autoimmune Conditions | Comments |
|---|---|---|---|---|
| IL-5 | Eosinophil survival | Reslizumab, Mepolizumab | Asthma | IgG1k mAbs |
| IL-17A | Inflammatory cytokine | Ixekizumab, Secukinumab | Psoriasis, Ankylosing Spondylitis | IgG4 vs. IgG1k |
| IL-23p19 | Inflammatory cytokine | Mirikizumab, Risankizumab | Ulcerative Colitis | Humanized IgG4 |
3.1 Innate Immune Deficiencies¶
Defects in phagocytosis (e.g., CGD), complement (e.g., C3 deficiency), or TLR signaling (e.g., TLR3/7/8/9) lead to recurrent infections and impaired microbial clearance.
3.2 Adaptive Immune Deficiencies¶
T-cell defects (e.g., SCID, WAS) and B-cell defects (e.g., XLA, CVID) result from mutations in genes like BTK, IL-7R α , or TACI, leading to impaired antibody production or T-cell function.
4. CLINICAL FEATURES¶
Clinical manifestations include recurrent infections (bacterial, viral, fungal), autoimmunity (e.g., hemolytic anemia, arthritis), and lymphoproliferation. Severe cases may present with opportunistic infections, chronic inflammation, or malignancies.
4.1 Infections¶
Common pathogens include Pneumocystis jirovecii, Staphylococcus aureus, and Mycobacteria. Infections are often severe, recurrent, or atypical (e.g., no pus in CGD).
4.2 Autoimmune and Inflammatory Complications¶
Autoimmune cytopenias, inflammatory bowel disease, and systemic vasculitis are common. IPEX syndrome presents with enteropathy, diabetes, and autoimmune manifestations.
5. DIFFERENTIAL DIAGNOSIS¶
Differential diagnoses include secondary immunodeficiencies (e.g., HIV, malnutrition), autoimmune diseases (e.g., lupus), and malignancies (e.g., lymphoma). Genetic testing and family history are critical for distinction.
5.1 Autoimmune Disorders¶
Conditions like lupus or rheumatoid arthritis may mimic PIDs but are typically associated with other systemic features (e.g., antinuclear antibodies).
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnostic workup includes clinical evaluation, laboratory tests (serum Ig levels, CH50/AP50), imaging (bone marrow biopsy), and genetic testing. Functional assays (e.g., DHR test for CGD) are essential.
Table 362-2: Diagnostic Tests for PIDs¶
| Test | Indication | Associated Conditions |
|---|---|---|
| Blood cell counts | Neutropenia, lymphopenia | SCID, CGD |
| Serum Ig levels | Hypogammaglobulinemia | XLA, CVID |
| CH50/AP50 | Complement deficiency | C3 deficiency, C5-C9 deficiencies |
| TREC analysis | SCID screening | Newborn screening |
6.1 Laboratory Tests¶
Key tests include serum IgG, IgA, IgM levels; CH50/AP50 for complement; TREC analysis for SCID; and functional assays (e.g., DHR, NBT) for phagocytic defects.
6.2 Imaging and Biopsy¶
Bone marrow biopsy and imaging (e.g., chest X-ray for thymic hypoplasia) help assess hematopoiesis and organ involvement.
7. MANAGEMENT & TREATMENT¶
Management includes immunoglobulin replacement, antimicrobial prophylaxis, and HSCT for curative therapy. Gene therapy and targeted therapies (e.g., IFN- γ for MSMD) are emerging options.
7.1 Immunoglobulin Replacement¶
IVIG or subcutaneous IgG is used for antibody deficiencies. Dosage is tailored to serum Ig levels and infection history.
7.2 HSCT and Gene Therapy¶
HSCT is curative for SCID and severe cases. Gene therapy is effective for X-linked SCID and ADA deficiency.
8. PROGNOSIS & COMPLICATIONS¶
Prognosis varies widely. Early diagnosis and treatment improve outcomes. Complications include recurrent infections, malignancies (e.g., lymphoma), and autoimmune diseases. Long-term follow-up is essential.
9. SPECIAL CONSIDERATIONS¶
Pregnancy in PIDs requires careful monitoring for maternal and fetal infections. Pediatric patients often present with recurrent infections, while elderly patients may have age-related immune decline. Genetic counseling is critical for families.
10. KEY POINTS & CLINICAL PEARLS¶
- PIDs are genetic disorders with Mendelian inheritance, requiring genetic testing for diagnosis.
- Early recognition and intervention (e.g., HSCT) are critical to prevent mortality.
- Immunoglobulin replacement and antimicrobial prophylaxis are mainstays of management.
- Differential diagnosis includes autoimmune and secondary immunodeficiencies.
- Multidisciplinary care involving immunologists, geneticists, and infectious disease specialists is essential.