||The nature of the immune system and different aspects of natural defense systems are classified. Structural analogies between hematopoietic and lymphopoietic tissues are defined. The interrelationships between humoral, cell-mediated, and complement-mediated immunity are covered in-depth with particular attention to the regulation of the immune response by cell-cell interactions. The mechanisms, mediation, and control of allergic reactions by immunomodulatory agents is discussed. Current concepts of autoimmunity are covered and the mechanisms and consequences of immunodeficiency and immunoproliferative disorders are discussed as well.