Types of Hypersensitivity Reactions

Why This Matters

Hypersensitivity reactions are what happens when the immune system's protective mechanisms cause harm instead of healing. You're being tested on your ability to distinguish which immune components drive each reaction type, how timing reveals mechanism, and why certain diseases cluster under specific hypersensitivity categories. These four types aren't arbitrary classifications; they reflect fundamentally different pathways involving distinct antibody classes, cell types, and effector mechanisms.

Don't just memorize that Type I involves IgE or that Type IV is T cell-mediated. Know why IgE-mast cell interactions produce immediate symptoms while T cell responses take 24-48 hours. Understand how the location of antigen (cell-bound vs. soluble vs. intracellular) determines which hypersensitivity pathway dominates. When an exam question describes a clinical scenario, you should be able to work backward from timing, symptoms, and mechanism to identify the reaction type.

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Antibody-Mediated Immediate Reactions

These reactions depend on preformed antibodies and occur rapidly because the immune system has already been primed. The speed of response correlates directly with whether effector molecules are ready and waiting.

Type I (Immediate) Hypersensitivity

IgE antibodies bound to mast cells are the central players here. During a first exposure to an allergen, $T_H2$ cells drive B cells to class-switch to IgE production. That IgE binds to high-affinity FcεRI receptors on mast cell surfaces, so by the time you encounter the allergen again, those mast cells are already "armed" and waiting.

• On re-exposure, the allergen cross-links two or more surface-bound IgE molecules, triggering mast cell degranulation. This releases preformed mediators (histamine, proteases) within minutes, followed by newly synthesized leukotrienes and prostaglandins. Together these cause vasodilation, bronchoconstriction, and increased vascular permeability.
• The clinical spectrum ranges from localized reactions (hay fever, hives, allergic asthma) to life-threatening anaphylaxis. The mechanism is the same across all of these; severity depends on allergen dose, route of entry, and how widely mast cells degranulate.
• A late-phase reaction can follow 4-8 hours later as eosinophils and other leukocytes are recruited by chemokines released during the initial degranulation. This is why asthma symptoms can return hours after the initial attack subsides.

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Antibody-Mediated Cytotoxic Reactions

When antibodies target antigens fixed to cell surfaces, the immune system destroys those cells as if they were pathogens. The key distinction from Type I is that the antigen is cell-bound, not soluble.

Type II (Antibody-Dependent) Hypersensitivity

IgG or IgM antibodies bind surface antigens on host cells, marking them for destruction through three main effector pathways:

1. Complement activation via the classical pathway, leading to formation of the membrane attack complex (MAC) and direct cell lysis
2. Opsonization, where complement fragments (especially C3b) and the Fc region of bound antibody coat the target cell, promoting phagocytosis
3. Antibody-dependent cellular cytotoxicity (ADCC), where NK cells recognize the Fc portion of bound IgG and kill the target cell

Transfusion reactions occur when recipient antibodies (e.g., anti-A or anti-B) attack donor red blood cells bearing incompatible surface antigens. Autoimmune hemolytic anemia involves the same destruction mechanism, but the antibodies target the patient's own erythrocytes. Goodpasture syndrome targets the basement membrane of glomeruli and alveoli, causing kidney and lung damage.

Coombs testing is the key diagnostic tool here. The direct Coombs test detects antibodies already bound to a patient's red blood cells. The indirect Coombs test detects free anti-RBC antibodies circulating in the patient's serum.

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Immune Complex-Mediated Reactions

When antigen-antibody complexes form in circulation and deposit in tissues, they trigger inflammation at sites far from the original antigen encounter. Deposition location determines which organs suffer damage.

Type III (Immune Complex) Hypersensitivity

Circulating immune complexes of IgG and soluble antigens deposit in blood vessel walls, joints, and kidney glomeruli. Once lodged in tissue, these complexes activate complement and generate C3a and C5a, which recruit neutrophils. The neutrophils then release lysosomal enzymes and reactive oxygen species, damaging the surrounding tissue rather than the original antigen.

• Systemic lupus erythematosus (SLE) exemplifies widespread complex deposition. Anti-dsDNA antibodies form complexes that deposit in kidneys (lupus nephritis), joints, skin, and serous membranes.
• Serum sickness shows the classic timeline: symptoms (fever, rash, arthralgia, glomerulonephritis) appear 7-10 days after a large antigen exposure, which is the time needed to mount an antibody response against the foreign protein.
• The Arthus reaction demonstrates localized Type III pathology. A subcutaneous injection of antigen in a previously sensitized individual causes local immune complex formation, complement activation, and neutrophil infiltration at the injection site within hours.

Whether complexes cause disease depends on their size and clearance. Small complexes stay soluble and are cleared by the liver and spleen. Very large complexes are quickly phagocytosed. It's the intermediate-sized complexes that evade clearance, circulate, and deposit in vulnerable vascular beds.

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Cell-Mediated Delayed Reactions

Type IV stands apart because it involves no antibodies. T cells are the sole mediators. The 24-48 hour delay reflects the time required for T cell activation, proliferation, and migration to the antigen site.

Type IV (Delayed-Type) Hypersensitivity

$CD4^+$ T helper cells (primarily $T_H1$) recognize processed antigen presented by macrophages and dendritic cells via MHC class II. Once activated, these T cells release cytokines, especially IFN-γ and TNF-α, which activate macrophages and amplify the inflammatory response. $CD8^+$ cytotoxic T cells can also contribute by directly killing antigen-bearing target cells.

• Contact dermatitis (poison ivy, nickel allergy) and the tuberculin skin test (PPD) are classic examples. Both require prior sensitization and show peak reactions at 48-72 hours. In the PPD test, induration (firm swelling from T cell and macrophage infiltration) is what you measure, not redness alone.
• Granuloma formation occurs in chronic Type IV responses when macrophages cannot eliminate the antigen. Persistent IFN-γ signaling causes macrophages to fuse into multinucleated giant cells and differentiate into epithelioid cells, forming a walled-off structure. This is seen in tuberculosis, sarcoidosis, and Crohn's disease.

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Quick Reference Table

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Self-Check Questions

1. A patient develops widespread urticaria and bronchospasm within 5 minutes of a penicillin injection. Which hypersensitivity type is this, and what antibody class is responsible?

2. Compare and contrast Type II and Type III hypersensitivity: both involve IgG, so what determines whether a reaction is classified as one versus the other?

3. A tuberculin skin test is read at 48 hours. Why would reading it at 4 hours give a false-negative result, and which immune cells are responsible for the positive reaction?

4. Which two hypersensitivity types would you consider if a patient presents with hemolytic anemia? How would you distinguish between them diagnostically?

5. Explain why anaphylaxis occurs within minutes while contact dermatitis takes days to develop. What mechanistic differences account for this timing distinction?