Immunoglobulins (Igs) or antibodies (Abs) are the principal operators of the adaptive humoral immune response. For optimum functional activity they acquire an optimized structure for antigen (Ag) recognition, precipitation, agglutination, phagocytosis (IgG1/3 and IgA), cytotoxicity (IgG1/3), transport through mucosa (IgA and IgM) and placenta (IgG1/3), complement activation (IgG1/3 and IgM) and release of inflammatory mediators (IgE). A diversity with potentially up to 10(15) different Ab specificities is generated during Ag-independent B cell development in the bone marrow by combinatorial V-D-J joining, creation of junctional diversity, and combinatorial association of L and H chains. Furthermore,Ab variety is created during Ag-dependent B cell maturation in peripheral lymphatic tissues by isotype class switching and somatic hypermutation. Two types of enzymes play a key role in Ab diverseness, i. e., the products of recombination-activating genes RAG1 and RAG2 and the affinity induced deaminase (AID). The prevailing adult-type B2 cells provide the basis for the acquired humoral immune response characterized by Ab production,Ag processing and presentation, immunological memory and tolerance along with the generation of the anti-idiotype network,whereas the fetal-type B1 cells may play a role in innate immunity and autoimmunity. Impairment of B cell immunity includes immunodeficiency (agammaglobulinemia), malignant transformation (leukemia, lymphoma, plasmocytoma) and immune dysregulation (allergy, autoimmunity). The diagnostic relevance of Abs comprises classical serology (immunoprecipitation, agglutination, complement binding, RIA, ELISA), immunocytochemistry and immunohistochemistry, immunofluorescence (microscopic and flow cytometric), cytotoxicity tests, immunoblots, immunospot assays and immunoabsorption (affinity chromatography). Therapeutic application of Abs (passive immunization) is directed against infections, intoxications, solid tumors, leukemias and lymphomas, graft rejection and graft-versus-host reaction, hemolytic anemia, and autoimmune diseases. The generation of genetically engineered monoclonal Abs (mAbs) has revolutionized the diagnostic and therapeutic potential of Abs in almost all disciplines of modern medicine.