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Immunology — Core — USMLE Step 2 CK Notes
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The four hypersensitivity reactions
- Type I (immediate, minutes): IgE, mast-cell degranulation — anaphylaxis, atopy
- Type II (antibody vs cell): IgG/IgM against surface antigen — haemolysis, Goodpasture (linear)
- Type III (immune complex): deposition and complement — lupus nephritis, serum sickness (granular)
- Type IV (delayed, days): T-cell mediated — contact dermatitis, TB skin test, transplant rejection
Immunodeficiencies by the missing compartment
- B-cell (Bruton, X-linked): recurrent encapsulated infections after ~6 months; absent B cells
- T-cell (DiGeorge, 22q11): tetany from hypocalcaemia, cardiac defects, absent thymus
- Combined (SCID): failure to thrive, opportunistic infection; adenosine deaminase deficiency
- Phagocyte (chronic granulomatous disease): catalase-positive organisms; abnormal dihydrorhodamine test
MHC and T-cell basics
- MHC I (all nucleated cells) presents to CD8; MHC II (antigen-presenting cells) presents to CD4
- Positive selection in the thymic cortex; negative selection in the medulla
- Superantigens cross-link MHC II to the T-cell receptor → massive cytokine release (toxic shock)
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