Antibodies are fundamental components of the immune system. Humans and other animals produce a vast repertoire of antibodies; sufficiently large to recognize and neutralize virtually any pathogen or toxin. This entails changes (mutations) of the antibody genes that happen in the B lymphocytes during the immune response. We focus on identifying and determining the function and regulation of the proteins that are part of the mechanisms mutating the antibody genes. The efficiency of these mechanisms determines the efficiency of the response and thereby, our health. Defects in these mechanisms cause immunodeficiency. Conversely, malfunctions lead to autoimmune diseases and cancer.