The original Gell and Coomb’s classification categorizes hypersensitivity reactions into four subtypes according to the type of immune response and the effector mechanism responsible for cell and tissue injury: type I, immediate or IgE mediated; type II, cytotoxic or IgG/IgM mediated; type III, IgG/IgM immune complex mediated; and type IV, delayed-type hypersensitivity or T-cell mediated. The classification has been improved so that type IIa is the former type II and type IIb is antibody-mediated cell stimulating (Graves Disease and the “autoimmune” type of chronic idiopathic urticaria). Type IV has four major categories: type IVa is CD4 Th1 lymphocyte mediated with activation of macrophages (granuloma formation and type I diabetes mellitus); type IVb is CD4 Th2 lymphocyte mediated with eosinophilic involvement (persistent asthma and allergic rhinitis); type IVc is cytotoxic CD8 T lymphocyte with involvement of perforin-granzme B in apoptosis (Stevens-Johnson syndrome and toxic epidermal necrolysis); type IVd is T-lymphocyte–driven neutrophilic inflammation (pustular psoriasis and acute generalized exanthematous pustulosis). Some diseases have multiple types of immunologic hypersensitivity.