Type IV Hypersensitivity (Delayed-Type / Cell-Mediated Hypersensitivity) — From Basics

1. What makes it different from Types I-III

Types I, II, and III hypersensitivity are all antibody-mediated (IgE, IgG, immune complexes) and occur within minutes to hours. Type IV is the odd one out:

Mediated by T lymphocytes, not antibodies
Delayed onset: 24-72 hours after antigen exposure (hence "delayed-type hypersensitivity," DTH)
Requires prior sensitization to the antigen
Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 205-206
Cellular and Molecular Immunology, p. 1259-1261

2. The two effector arms

a) CD4+ T cell-mediated inflammation (classic DTH)

Antigen is presented by dendritic cells/APCs to naive CD4+ T cells, which secrete IL-2 (autocrine growth factor) and proliferate
Depending on cytokines from the APCs, CD4+ cells differentiate into:
- Th1 cells (driven by IL-12) → secrete IFN-γ on re-exposure → this "classically activates" macrophages, boosting their microbicidal killing, MHC II expression, and further inflammatory cytokine output. Reaction dominated by activated macrophages.
- Th17 cells (driven by IL-1, IL-6, IL-23) → recruit neutrophils — this arm is more prominent in autoimmune tissue injury.
Some effector cells persist as long-lived memory T cells, ready to react faster on re-exposure.

b) CD8+ cytotoxic T cell (CTL)-mediated killing

CD8+ CTLs directly kill antigen-bearing target cells (via perforin/granzyme and Fas-FasL pathways)
Dominant mechanism in many viral infections and some autoimmune/graft-rejection settings
Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 205

3. Two-step process: Sensitization then Elicitation

The classic model (originally shown in guinea pigs, directly applicable to humans):

Sensitization phase: first exposure to antigen (via infection, contact with a chemical, or injection) primes T cells — this takes about 2 weeks.
Elicitation (challenge) phase: re-exposure to the same antigen triggers the visible reaction.

Timeline of the reaction after challenge:

Time	Event
~4 hours	Neutrophils accumulate around postcapillary venules at the site
~12 hours	T cells and monocytes infiltrate, forming a perivascular cuff; venule endothelium swells and becomes leaky
18-48 hours	Fibrinogen leaks out and is converted to fibrin; edema + fibrin + T cell/monocyte accumulation cause induration (firm swelling) — the hallmark diagnostic sign, maximal at 24-48 hours

Cellular and Molecular Immunology, p. 1259-1261

4. The prototype clinical example: the Tuberculin (Mantoux) test

Purified protein derivative (PPD) of Mycobacterium tuberculosis is injected intradermally into a person previously exposed to TB. A positive DTH reaction (induration) at 48-72 hours indicates prior/latent TB exposure. Loss of this response to universal recall antigens (e.g., Candida) indicates anergy — a sign of deficient T cell function (important in evaluating immunocompromised patients).

Cellular and Molecular Immunology, p. 1260

5. Clinical examples to remember

Contact dermatitis: poison ivy/oak (urushiol modifies self proteins → neoantigen), nickel, chromium, thiuram (latex gloves), topical drugs. Both CD4+ and CD8+ T cells can be involved; chronic forms are called eczema.
Granulomatous inflammation: when macrophages cannot clear the antigen (e.g., persistent intracellular M. tuberculosis, M. leprae, foreign material, sarcoidosis), continuous Th1/macrophage activation forms epithelioid cell granulomas with giant cells.
Autoimmune diseases driven predominantly by T cell-mediated tissue injury:
- Rheumatoid arthritis (Th17-mediated joint inflammation)
- Multiple sclerosis (Th1/Th17 against myelin antigens → demyelination)
- Type 1 diabetes mellitus (T cells against islet β-cell antigens)
Transplant/graft rejection (T cell-mediated component)
Some drug hypersensitivity reactions (e.g., certain drug rashes, Stevens-Johnson syndrome/TEN are T cell-mediated)
Robbins, Cotran & Kumar Pathologic Basis of Disease, Table 6.5, p. 206
Katzung's Basic and Clinical Pharmacology, 16th Ed., p. 752-759

Quick summary table

Feature	Type IV Hypersensitivity
Mediator	T cells (CD4+ Th1/Th17, CD8+ CTLs) — no antibody
Onset	24-72 hours (delayed)
Key cytokine	IFN-γ (Th1) activates macrophages
Classic test	Tuberculin/PPD skin test
Histology	Perivascular T cell/monocyte cuffing, induration, granulomas if antigen persists
Examples	TB skin test, contact dermatitis, granulomas (TB, leprosy, sarcoid), MS, RA, T1DM, graft rejection

One current clinical extension worth flagging: a 2025 EAACI position paper on drug hypersensitivity to antiplatelet agents (aspirin, clopidogrel) discusses T cell-mediated (delayed) mechanisms in some antiplatelet drug reactions and management via desensitization protocols (PMID: 40704552) — useful if you want to see Type IV mechanisms applied to a modern pharmacology context.

If you want, I can go deeper into any one branch next — e.g., granulomatous inflammation in TB, contact dermatitis mechanism in detail, or how DTH differs practically from Type II/III in exam-style comparison tables.

This is a shared conversation. Sign in to Orris to start your own chat.