Hypersensitivity reactions

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infections and foreign antigens. Another form of delayed hypersensitivity is contact dermatitis(poison ivy, chemicals, heavy metals, etc.) in which the lesions are more papular. Type IVhypersensitivity can be classified into three categories depending on the time of onset and clinicaland histological presentation (Table 3).Mechanisms of damage in delayed hypersensitivity include T lymphocytes and monocytes and/ormacrophages. The pathogenesis is triggered primarily by helper T (TH1) cells that secretecytokines that activate and recruit macrophages, which cause the bulk of the damage (Figure 3).The delayed hypersensitivity lesions mainly contain monocytes and T cells.Major lymphokines involved in delayed hypersensitivity reaction include monocyte chemotacticfactor, interleukin-2, interferon-γ, TNF α, etc.Table 3. Delayed hypersensitivity <strong>reactions</strong>typereactiontimeclinicalappearancecontact 48-72 hr eczematuberculingranuloma48-72 hr21-28dayslocalindurationhardeninghistologylymphocytes, followed bymacrophages; edema ofepidermislymphocytes, monocytes,macrophagesmacrophages, epitheloidand giant cells, fibrosisantigen and siteepidermal ( organicchemicals, poison ivy,heavy metals, etc.)intradermal (tuberculin,lepromin, etc.)persistent antigen orforeign body presence(tuberculosis, leprosy, etc.)Diagnostic tests in vivo include delayed cutaneous reaction (e.g. Montoux test) and patch test (forcontact dermatitis). In vitro tests for delayed hypersensitivity include mitogenic response,lympho-cytotoxicity and IL-2 production.Corticosteroids and other immunosuppressive agents are used in treatment.A comparative summary of all four types of hypersensitivity <strong>reactions</strong> have been presented inTable 4 below.6
Table 4. Comparison of Different Types of hypersensitivitycharacteristicstype-I(anaphylactic)type-II(cytotoxic)type-III(immunecomplextype-IV(delayed type)antibodyIgEIgG, IgMIgG, IgMNoneantigenexogenouscell surfacesolubletissues & organsresponse time15-30 minutesminutes-hours3-8 hours48-72 hoursappearancewheal & flarelysis and necrosiserythema andedema, necrosiserythema andindurationhistologybasophils andeosinophilantibody andcomplementcomplement andneutrophilsmacrophages andT cellstransferred withantibodyantibodyantibodyT-cellsexamplesallergic asthma,hay fevererythroblastosisfetalis, Goodpasture’snephritisSLE, farmer’slung diseasetuberculin test,poison ivy,granuloma7
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Hypersensitivity reactions

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