Publisher Summary This chapter presents the laboratory models and experimental findings of immediate hypersensitivity. The two major types of responses, as comprising the totality of the immune response, are the immediate type response and the delayed type response. The immediate type reactions can be categorized by the nature of the antigen and the immunoglobulin class of the antibody initiating the response. Responses initiated by IgG and IgM antibodies usually result in the activation of a series of plasma proteins, which act in cascade fashion. The persistence of IgE in the skin has given this immunoglobulin its descriptive name of homocytotropic antibody. Serial dilutions of the antibody solution are injected into the skin of recipient animal. The passive cutaneous anaphylaxis reaction, which results is due to the diffusion of the antigen out of the vasculature, and the combining of the antigen with the tissue-bound antibody molecules causing the release of the pharmacologic mediators from the tissue mast cells. In vitro methods for studying cell sensitization and release of pharmacologic mediators offer much better control over animal to animal variation. Sensitive methods are available for the quantitation of the amounts of the various pharmacologic mediators of anaphylaxis, which are released during anaphylactic challenge.