Master Immunology
for USMLE Step 1

What the USMLE Step 1 Tests in Immunology

Immunology on USMLE Step 1 tests your ability to link immune mechanisms to clinical presentations, laboratory findings, and therapeutic decisions. You must identify primary immunodeficiencies (e.g., DiGeorge syndrome, CVID, chronic granulomatous disease) based on recurrent infections, age at onset, and pathogen types. Hypersensitivity reactions are assessed through clinical scenarios (e.g., anaphylaxis, serum sickness, contact dermatitis). Autoimmune diseases like SLE, rheumatoid arthritis, and Goodpasture syndrome require recognition of autoantibodies, diagnostic criteria (e.g., ACR criteria for SLE), and first-line treatments (e.g., hydroxychloroquine, corticosteroids). Transplant rejection patterns (hyperacute, acute, chronic) and immunosuppressive drugs (cyclosporine, tacrolimus, mycophenolate) are tested. Vaccination schedules, immunisation in immunocompromised hosts, and monoclonal antibody therapies (e.g., rituximab, infliximab) are high-yield. Complement deficiencies and their associations (e.g., C5-9 deficiency and Neisseria infections) are frequently examined.

High-Yield Concepts

• Primary Immunodeficiencies: DiGeorge syndrome: 22q11.2 deletion, thymic aplasia → T-cell deficiency, hypocalcemia, cardiac defects. Bruton agammaglobulinemia: X-linked, Btk mutation, absent B cells and immunoglobulins, recurrent bacterial infections after 6 months. CVID: low IgG and IgA, recurrent sinopulmonary infections, autoimmune cytopenias; treat with IVIG. Chronic granulomatous disease: NADPH oxidase defect, recurrent catalase-positive infections (Staph aureus, Aspergillus), negative NBT test. Hyper-IgM syndrome: CD40L deficiency, normal IgM with low IgG/IgA/E, opportunistic infections (Pneumocystis).
• Hypersensitivity Reactions: Type I (IgE): anaphylaxis, allergic rhinitis, asthma; first-line: epinephrine for anaphylaxis, antihistamines. Type II (IgG/IgM): Goodpasture syndrome (anti-GBM antibodies), autoimmune hemolytic anemia (anti-RBC antibodies), myasthenia gravis (anti-AChR). Type III (immune complex): SLE (anti-dsDNA), serum sickness (heterologous proteins), post-streptococcal glomerulonephritis. Type IV (T-cell mediated): contact dermatitis (poison ivy), PPD test, graft-versus-host disease.
• Autoimmune Diseases: SLE and Rheumatoid Arthritis: SLE diagnosis: requires ≥4 of 11 ACR criteria (e.g., malar rash, discoid rash, photosensitivity, oral ulcers, arthritis, serositis, renal disorder, neurologic disorder, hematologic disorder, immunologic disorder [anti-dsDNA, anti-Sm], antinuclear antibody). First-line: hydroxychloroquine, corticosteroids for flares, mycophenolate for lupus nephritis. Rheumatoid arthritis: anti-CCP and rheumatoid factor, symmetric small joint arthritis, morning stiffness >30 min; treat with methotrexate, TNF inhibitors (adalimumab, etanercept).
• Transplant Immunology: Hyperacute rejection: preformed anti-HLA antibodies, minutes to hours, treat with removal of graft. Acute rejection: T-cell mediated, days to weeks, treat with high-dose corticosteroids, anti-thymocyte globulin. Chronic rejection: antibody-mediated, months to years, leads to fibrosis; no effective treatment. Immunosuppressants: calcineurin inhibitors (cyclosporine, tacrolimus) inhibit IL-2 transcription; mycophenolate mofetil inhibits purine synthesis; sirolimus inhibits mTOR.
• Complement System: Classical pathway: C1q, C1r, C1s, C4, C2; activated by IgG/IgM immune complexes. Alternative pathway: factor B, D, properdin; activated by microbial surfaces. Lectin pathway: mannose-binding lectin. Deficiencies: C1q deficiency → SLE-like disease; C3 deficiency → severe pyogenic infections; C5-9 deficiency → recurrent Neisseria meningitidis infections; C1 esterase inhibitor deficiency → hereditary angioedema (treat with icatibant or fresh frozen plasma).
• Monoclonal Antibodies and Immunomodulators: Rituximab: anti-CD20, used for B-cell lymphomas, rheumatoid arthritis, ITP. Infliximab: anti-TNF, used for Crohn disease, rheumatoid arthritis, ankylosing spondylitis; risk of reactivation of latent TB. Trastuzumab: anti-HER2 for breast cancer. Bevacizumab: anti-VEGF for colorectal cancer. Omalizumab: anti-IgE for severe allergic asthma. Checkpoint inhibitors (ipilimumab, anti-CTLA-4; nivolumab, anti-PD-1) used in melanoma, lung cancer; side effects: immune-related adverse events (colitis, pneumonitis).
• Vaccination and Immunization: Live attenuated vaccines: MMR, varicella, yellow fever, oral polio, nasal influenza; contraindicated in pregnancy and severe immunodeficiency (e.g., SCID, HIV with CD4 <200). Inactivated vaccines: IPV, influenza (injectable), hepatitis A, rabies. Recommended schedule: HepB at birth, DTaP at 2,4,6 months, MMR at 12-15 months and 4-6 years. Post-exposure prophylaxis: rabies vaccine and immunoglobulin; tetanus toxoid for dirty wounds if >5 years since last dose.
• Laboratory Immunology: ELISA: detect antigen or antibody using enzyme-linked reaction; used for HIV screening (ELISA → Western blot confirm). Flow cytometry: CD4 count for HIV staging (treatment threshold <350 cells/μL, AIDS <200). Western blot: confirmatory test for HIV, Lyme disease. Serum protein electrophoresis: monoclonal gammopathy (multiple myeloma: M spike, immunoparesis). Antinuclear antibody (ANA): sensitive for SLE (positive in >95%), but not specific. Anti-dsDNA: specific for SLE, correlates with disease activity.

Common Traps in Immunology Questions

• Confusing Bruton agammaglobulinemia (X-linked, B-cell defect) with severe combined immunodeficiency (SCID, T- and B-cell defect, presents earlier with opportunistic infections).
• Thinking that a positive ANA confirms SLE — it is sensitive but not specific; anti-dsDNA and anti-Sm are more specific.
• Forgetting that complement deficiencies (especially C5-9) predispose specifically to Neisseria infections, not all encapsulated bacteria.
• Assuming that all monoclonal antibodies are immunosuppressive — trastuzumab and bevacizumab are not immunosuppressive; they target growth factors.
• Mixing up Type III hypersensitivity (immune complex deposition, e.g., serum sickness) with Type II (antibody against cell surface receptors, e.g., Goodpasture).
• Believing that live vaccines are safe in all immunocompromised patients — they are contraindicated in severe primary immunodeficiencies and during high-dose immunosuppressive therapy.

How to Revise Immunology for the USMLE Step 1

Focus on linking immune defects to clinical scenarios: a child with recurrent sinopulmonary infections and low immunoglobulins points to CVID or Bruton agammaglobulinemia; a patient with recurrent Neisseria meningitis suggests complement deficiency. Memorise the ACR criteria for SLE and the diagnostic autoantibodies for autoimmune hepatitis, primary biliary cholangitis, and Goodpasture syndrome. Practice interpreting immunology lab results (ELISA, Western blot, flow cytometry, complement levels). Questions often present a clinical vignette with a laboratory finding and ask for the most likely diagnosis, next step in management, or mechanism of a drug. Prioritise understanding hypersensitivity mechanisms and transplant rejection types. Review monoclonal antibody names and their targets (e.g., -ximab, -zumab, -mab suffixes). Use spaced repetition for immunodeficiency syndromes and their associated pathogens.

Practise it: MedLumen has 30 Immunology questions for the USMLE Step 1, each with a full explanation and references.