Injury or foreign invasion will instigate a cascade of events directed at eliminating the intruder and augmenting the healing process. This involves the unification of two separate processes (inflammatory and immune processes) to provide an effective host defence. Chemical mediators converge on the site of tissue damage and exert local and distant effects. The immune response is divided into innate and acquired immunity. The immediate, non-specific innate response, combined with the specifically targeted acquired response, provide our major defence mechanisms. Lymphocytes and immunoglobulins are the hallmark of acquired immunity. Regulation of these interlinked systems provide cohesion and a group of soluble proteins called cytokines have a major role. Protective immune mechanisms can sometimes cause detrimental effects to the host. We discuss and classify allergic reactions, in particular, the most severe and potentially life threatening form – anaphylaxis.