The immune system.
D000917:Antibody Formation; D000937:Antigen-Antibody Reactions; D001853:Bone Marrow / Q000276:immunology; D001854:Bone Marrow Cells; D005547:Foreign Bodies; D006801:Humans; D007110:Immunity, Active; D008198:Lymph Nodes / Q000276:immunology; D008208:Lymphatic System / Q000276:immunology*; D008214:Lymphocytes / Q000276:immunology*; D008961:Models, Structural; D009424:Nervous System Physiological Phenomena; D013154:Spleen / Q000276:immunology; D013950:Thymus Gland / Q000276:immunology