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Hypersensitivity reactions, autoimmunity and an immunodeficiency example
Hypersensitivity is divided into four types by mechanism — type I mediated by IgE and mast cells, type II antibodies against an antigen on a cell or in tissue, type III immune complexes deposited in vessels, and type IV T cells — and recognising the mechanism explains the histological picture and the tests.
Why it matters in the exam
The bank asks the student to assign a disease to a mechanism. Type I: anaphylaxis, allergic asthma, allergic rhinitis — an immediate reaction from histamine and a late-phase reaction with eosinophils hours later. Type II: autoimmune haemolytic anaemia, acute transfusion reaction, Goodpasture syndrome (antibodies against type IV collagen in the basement membrane), pemphigus, myasthenia gravis and Graves disease (the last two through altered receptor function), and rheumatic fever — in which antibodies against group A streptococcus cross-react with heart tissue (molecular mimicry).
Type III: systemic lupus erythematosus, post-streptococcal glomerulonephritis, serum sickness and the Arthus reaction; the deposits appear granular on immunofluorescence, with inflammation and fibrinoid necrosis. In lupus, ANA is very sensitive but not specific, and antibodies against double-stranded DNA and Sm are specific. Type IV: contact dermatitis, the tuberculin reaction, granulomas, type 1 diabetes and multiple sclerosis — a reaction appearing after 48–72 hours.
An immunodeficiency example the bank asks about: X-linked agammaglobulinaemia (Bruton) — a mutation in BTK, absent B cells and antibodies, and small tonsils and nodes; bacterial infections begin after 6 months of age when maternal antibodies fall, and T cells are normal. Graft rejection uses the same mechanisms: hyperacute rejection from pre-existing antibodies, acute rejection from T cells or antibodies, and chronic rejection with vascular narrowing.
Key points
- I: IgE; II: antibodies against cells; III: complexes; IV: T cells.
- Goodpasture — linear; lupus — granular.
- Lupus: ANA sensitive; anti-dsDNA and anti-Sm specific.
- Type IV appears after 48–72 hours.
- Bruton: BTK, no B cells, infections from 6 months.
Summary table
| Type | Mechanism | Examples | Test or finding |
|---|---|---|---|
| I | IgE and mast cells | Anaphylaxis, allergic asthma | Tryptase, specific IgE |
| II | Antibodies against cell or tissue | Goodpasture, AIHA, myasthenia, Graves, rheumatic fever | Direct Coombs; linear deposit |
| III | Immune complexes | Lupus, post-streptococcal glomerulonephritis, serum sickness | Granular deposit; low complement |
| IV | T cells | Contact dermatitis, tuberculin, type 1 diabetes | Reaction after 48–72 hours |
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Sources
- Robbins and Cotran Pathologic Basis of Disease, 10th edition (2021) — Diseases of the immune system — hypersensitivity and autoimmunity
- Abbas AK, Lichtman AH, Pillai S. Cellular and Molecular Immunology, 10th edition (2022) — Hypersensitivity disorders; congenital immunodeficiencies
- EULAR/ACR classification criteria for systemic lupus erythematosus, 2019 — Immunological domains
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.