Autoimmunity and Allergy: Same Breach, Different Effector | AI Learning Guide

The comparison table separates the two conditions by mechanism, not by symptom. Read the autoimmunity column first: the antigen is a self molecule, the effectors are self-reactive T cells and IgG autoantibodies, and the damage uses complement, Fc receptors, and direct cytotoxicity. Because the self antigen never goes away, the inflammation is chronic. Now read the allergy column: the antigen is a harmless environmental protein, and the effector arm is Th2 cells, IgE, and mast cells. The mechanism here is worth walking through. On first exposure, allergen-specific B cells class-switch to IgE under IL-4 and IL-13 from Th2 cells. That IgE binds the high-affinity Fc-epsilon receptor on mast cells, arming them. On re-exposure, allergen cross-links the bound IgE, and the mast cell degranulates within minutes — histamine, leukotrienes, prostaglandins. A late phase follows as eosinophils and Th2 cells infiltrate. The list below shows that this is only type I; types II, III, and IV use different effectors. The key point: both conditions are failures of restraint, not failures of defense.

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Autoimmunity versus allergy by mechanism

Autoimmunity

Antigen: self molecule (beta cell, myelin, synovium, thyroid)
Effector arm: self-reactive CD4/CD8 T cells and IgG autoantibodies
Tissue damage: complement, Fc receptors, direct cytotoxicity, cytokine-driven inflammation
Time course: chronic, because the self antigen is always present
Example: type 1 diabetes, myasthenia gravis, Graves disease

Allergy

Antigen: harmless environmental protein (pollen, food, drug hapten)
Effector arm: Th2 cells, IgE, mast cells, eosinophils
Tissue damage: IgE cross-linking triggers mast cell degranulation; histamine, leukotrienes, prostaglandins
Time course: immediate within minutes, then a late phase over hours
Example: allergic rhinitis, food anaphylaxis, asthma

Hypersensitivity types by effector

Type I: IgE and mast cells — immediate hypersensitivity.
Type II: IgG against cell-bound or matrix-bound antigen — complement and Fc receptor mediated.
Type III: immune complexes deposited in tissue — complement activation and neutrophil recruitment.
Type IV: T-cell mediated, delayed — macrophage activation and cytotoxic T-cell killing.

Both conditions reflect regulatory failure rather than effector failure. The same patient can develop both, and defects in Treg number or function predispose to both, which supports a shared underlying tolerance defect rather than two independent diseases.