Understanding the pathogenesis of immunologic disease, i.e., the way in which immunologic events cause injury and/or malfunction, demands several kinds of information. First, one must identify the particular antigen(s) which initiates and serves as the target of the pathogenic immune response. This is often a difficult task, since many of these antigens are substances normally present or at least commonly present in the absence of an immune response or disease. This fact suggests that transient events such as incidental infections or exposures to environmental agents may have an adjuvant effect which increases immunogenicity of potential antigens or in other ways terminates the tolerance barrier. Second, one must know a number of things about the host which are important in determining the self-infliction of immunologic injury. It appears that the magnitude and perhaps balance, i.e., cellular vs humoral aspects, of the immune response, are in part genetically predetermined and therefore may predispose to immunologic disease.