Immunopathology is important for bodys immune system and associated diseases. Primary and secondary immunodeficienies are common diseases and early diagnosis of diseases like AIDS, lymphoma, leukaemia, malignancies are very important for management of these diseases. This chapter deals with active and passive immunity, commonly used terms in immunology, biological activities, source of different classes of immunoglobulin (IgG, IgM, IgA, IgD, IgE), differences between B and T lymphocytes, cells of immune system-B cell, T cell, NK cell, macrophages, CD4+, CD8+ cells, HLA antigen and its association with diseases. Types of hypersensitivity Reactions Type I, Type II, Type III, Type IV, AIDS its etiopathogenesis, clinical manifestations, diagnosis, transplant rejection, mechanism of graft rejection, types of antigen antibody tests like agglutination, precipitation, CFT Radioimmunoassay their principle, interpretation and indication are described.