The present knowledge of 4 areas of immunology applied to infectious diseases were reviewed. These areas were immunodiagnostic, immunoresponse of the host, immunopathology and immunizations. The immune mechanisms against the different pathogenic agents are practically the same, since the immunological system in one and the response varies only depending on the chemical composition, the different structures and the virulence of the immunogens present in the invading microorganisms. In regard to immunodiaqnosis. the characteristics of the different serological tests and the hypersensitivity response in the study of the infectious agents were commented. Dealing with the immune response of the host, the different biological mechanisms used against the aggression of the invading microorganisms were considered. On the subject of immunopathology, the types of organic alterations produced by the immune response, that harms the host instead of protecting it, are discussed. Finally, reference is made to the importance of immunizations as a valuable method for protecting the host against infections agents.