Hypersensitivity Reactions (Types I-IV)

A farmer gets stung by a bee. Within minutes he develops diffuse urticaria, throat swelling, and hypotension. Which antibody is mediating this disaster?

IgG — it's the most abundant, makes sense

IgE — mast cell degranulation

IgM — first responder

IgA — mucosal surfaces

IgE. It's always IgE for anaphylaxis. IgE sits on mast cells like a loaded gun. Second exposure = trigger pull = massive histamine dump. Minutes. Not hours. Not days. Minutes.

Not quite. Anaphylaxis = Type I = IgE. IgE is pre-loaded on mast cells. When the antigen cross-links two IgE molecules, the mast cell explodes — histamine, leukotrienes, prostaglandins, everything. That's why it's so fast.

The Four Ways to Overreact

Your immune system has four distinct modes of going too far. Boards want you to know which mechanism produces which disease. The secret: think about the timing and the mediator.

Types I-III use antibodies (humoral). Type IV uses T-cells (cell-mediated). That's your first fork. 🔑ABC = Antibodies for types 1-2-3. Type 4 = 4gotten antibodies (T-cells only).

Type I — Immediate (Anaphylactic)

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Antibody: IgE

Timing: Minutes — this is the fast one

Mechanism: Antigen cross-links IgE on mast cellsTissue-resident immune cells loaded with granules of histamine, heparin, and proteases. They sit in connective tissue near blood vessels and at body surfaces — stationed at the borders, waiting for the signal to explode. → degranulation → histamine flood

Two phases:

Early (minutes): Preformed mediators — histamine, tryptase. Vasodilation, bronchospasm, edema.
Late (4-8 hours): Newly synthesized — leukotrienes, prostaglandins, cytokines. The second wave that can kill you after you thought you were fine.

Classic diseases:

Anaphylaxis (bee stings, peanuts, penicillin)
Allergic rhinitis (hay fever)
Allergic asthma
Urticaria (hives)
Food allergies

Board Trap

Anaphylaxis can have a biphasic reaction. Patient gets epi, feels better, goes home — then crashes 4-8 hours later from the late phase. That's why you observe for hours after anaphylaxis. Boards love this.

Type II — Cytotoxic (Antibody-Mediated)

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Antibody: IgG or IgM

Timing: Hours to days

Mechanism: Antibodies bind to antigens on the cell surface → cell gets destroyed by complement or NK cells (ADCC). The target is the cell itself.

Three subtypes (boards love these):

Cytotoxic: Antibody + complement = cell lysis. Autoimmune hemolytic anemia, Rh disease of newborn
Opsonization: Antibody flags cell for phagocytosis. ITP (platelets get eaten)
Functional: Antibody binds receptor — stimulates OR blocks it.
- Graves' disease — TSIThyroid-Stimulating Immunoglobulin. An IgG that mimics TSH by binding and activating the TSH receptor. The thyroid can't tell the difference — it just keeps making hormone. Graves' = hyperthyroid despite low TSH. stimulates TSH receptor → hyperthyroidism
- Myasthenia Gravis — antibody blocks AChRAcetylcholine Receptor at the neuromuscular junction. Anti-AChR antibodies prevent acetylcholine from binding — muscles don't get the signal. Weakness worsens with use because the few remaining receptors fatigue. → weakness

Disease list:

Autoimmune hemolytic anemia
Rh hemolytic disease (erythroblastosis fetalis)
ABO transfusion reactions
Goodpasture syndrome (anti-GBM → linear IF)
Graves' disease (stimulatory)
Myasthenia Gravis (blocking)
Pemphigus vulgaris (anti-desmoglein)
Rheumatic fever (molecular mimicry)
Hyperacute transplant rejection
Drug-induced: penicillin (hapten on RBCs → IgG → hemolysis), methyldopa (induces autoantibodies against RBC Rh antigens)

🔑Type II = antibody attacks the cell surface directly. Think: "2 close for comfort" — the antibody is RIGHT ON the cell.

Type III — Immune Complex

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Antibody: IgG (sometimes IgM)

Timing: Hours to weeks

Mechanism: Antibodies bind soluble antigens floating in blood → form immune complexes → complexes deposit in tissues (joints, kidneys, vessels) → complement activation → neutrophil recruitmentComplement fragments C3a and C5a are chemoattractants. Neutrophils arrive and release proteases and reactive oxygen species trying to destroy the complexes — but the tissue gets caught in the crossfire. That's the damage. → tissue damage

Key difference from Type II: Type II = antibody binds cell surface antigen (fixed). Type III = antibody binds soluble antigen (floating, clumps, deposits wherever blood flow slows).

Classic diseases:

SLE — anti-dsDNA complexes deposit in kidneys, joints, skin
Serum sickness — foreign protein → fever, urticaria, arthralgia, lymphadenopathy 7-14 days later
Arthus reaction — localized Type III at injection site
PSGN — post-streptococcal glomerulonephritis1-3 weeks after Group A Strep infection. Immune complexes deposit in glomeruli → complement activation → "lumpy-bumpy" granular pattern on IF. Classic: kid with periorbital edema, cola-colored urine, high ASO titers.
Polyarteritis nodosa — Hep B immune complexes in vessel walls

Board Trap: Serum Sickness vs Serum Sickness-LIKE

Serum sickness = actual immune complexes (Type III). Caused by foreign proteins (antithymocyte globulin, infliximab).

Serum sickness-LIKE = clinically identical but caused by drugs (penicillin, cefaclor). No immune complex deposits on biopsy. Boards test this distinction.

Type IV — Delayed (Cell-Mediated)

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Antibody: NONE — T-cells only

Timing: 48-72 hours (the slow burn)

Mechanism: Sensitized T-cells encounter antigen → release cytokines → macrophage activation → inflammation. No antibodies involved.

Two flavors:

CD4+ T-helper (Th1): IFN-γ → activate macrophages → granuloma formation. TB, sarcoidosis, Crohn's.
CD8+ cytotoxic: Directly kill target cells. Contact dermatitis, transplant rejection.

Classic diseases:

TB skin test (PPD) — read at 48-72 hours. The board classic.
Contact dermatitis — poison ivy, nickel, latex
GVHD — donor T-cells attack host
Acute transplant rejection
Multiple sclerosis — T-cells attack myelin
Type 1 diabetes — T-cells destroy beta cells
Hashimoto's thyroiditis
Granulomatous diseases — TB, sarcoidosis, Crohn's, berylliosis

Board Trap: PPD Timing

TB skin test is read at 48-72 hours. Not 24. Not immediately. Measure the induration (not redness). If they ask when to read the PPD, the answer is always 48-72h because Type IV = delayed.

Side-by-Side Comparison

Feature	Type I	Type II	Type III	Type IV
Mediator	IgE	IgG/IgM	IgG/IgM	T-cells
Timing	Minutes	Hours-days	Hours-weeks	48-72 hrs
Target	Mast cells	Cell surface Ag	Soluble Ag	Tissue cells
Complement?	No	Yes	Yes	No
Key cells	Mast cells, basophils	NK cells, macrophages	Neutrophils	Macrophages, T-cells
IF pattern	N/A	Linear	Granular	N/A
Buzzword	"Anaphylaxis"	"Autoantibody"	"Immune complex"	"Granuloma"
Board classic	Bee sting → epi	Graves'/MG	SLE nephritis	PPD at 48-72h

The Hard One: Type II vs Type III

Both use IgG. Both activate complement. Boards will test whether you know the difference.

Type II: Antibody binds antigen stuck to a cell surface. The cell is the target. → Linear IF

Type III: Antibody binds antigen floating free in blood. They clump and deposit in tissues. → Granular IF

Type II is a targeted assassination (antibody finds the cell and kills it). Type III is collateral damage (immune complexes rain down wherever they land).

Goodpasture syndrome — antibodies attack the glomerular basement membrane. Type II or Type III?

Type II — antibody binds directly to the GBM (a fixed tissue component)

Type III — it's in the kidney, so it must be immune complex deposition

Type II. Anti-GBM antibodies bind directly to collagen IV in the basement membrane. Fixed target. On IF: linear staining. Type III shows granular ("lumpy-bumpy") because complexes deposit randomly.

Good instinct — kidneys do get hit by Type III (lupus nephritis). But Goodpasture is Type II. Anti-GBM binds directly to a fixed component. The tell: linear IF = Type II. Granular IF = Type III.

Sort the Diseases

Tap a disease, then tap the type it belongs to. 16 diseases. Go.

Type I

Type II

Type III

Type IV

Clinical Decision Tree

Patient has a suspected hypersensitivity reaction. Walk through it.

Does the reaction involve antibodies or T-cells?

Antibodies (serology positive, complement activated)

T-cells (delayed onset, granulomas, negative serology)

Elimination Game

5 scenarios. Each clue eliminates one type. Last one standing wins. Or just guess early if you're feeling brave.

Quiz Time

4 patients with immune systems that can't chill. Don't make it worse. Different questions every reload.

Every wrong answer teaches more than every right one.