humoral immune response defends primarily against extracellular bacteria and viral infections.The cells involved in both immune systems are lymphocytes which originate in the bone marrow and migrate to different lymphoid organs.There are two types of lymphocytes which are known as T cells and B cells.The two types of lymphocytes are responsible for a dual immunity phases Figure 1.All lymphocytes are derived from bone marrow but those that pass through the thymus become T cells.(T lymphocytes).The T cells are responsible for cell-mediated immunity.Since the immunity involving T cells is associated with the T cells themselves,this type of immunity is called cell-mediated immunity. Other lymphocytes pass through the bursa of Fabricius in birds or the bursa equivalent in mammals and become B cells (B lymphocytes).The bursa of Fabricius is the primary lymphoid organ associated with the cloaca in birds but is not found in mammals.The bursa-derived lymphocytes(B cells)produce antibodies which can react specifically with antigen.Because B cells produce antibodies that circulate and the immunity is called humoral immunity.Antigen specific antibodies produced in the humoral immune response can be used in immunological assays to detect various disease agents or antigenic molecules.With the development of monoclonal antibodies,greater specificity and reproducibility can be obtained with immunological assays.The remainder of this discussion will be devoted to the humoral immune response. The Humoral Immune Response humoral immune response defends primarily against extracellular bacteria and viral infections. The cells involved in both immune systems are lymphocytes which originate in the bone marrow and migrate to different lymphoid organs. There are two types of lymphocytes which are known as T cells and B cells. The two types of lymphocytes are responsible for a dual immunity phases Figure 1. All lymphocytes are derived from bone marrow but those that pass through the thymus become T cells. (T lymphocytes). The T cells are responsible for cell-mediated immunity. Since the immunity involving T cells is associated with the T cells themselves, this type of immunity is called cell-mediated immunity. Other lymphocytes pass through the bursa of Fabricius in birds or the bursa equivalent in mammals and become B cells (B lymphocytes). The bursa of Fabricius is the primary lymphoid organ associated with the cloaca in birds but is not found in mammals. The bursa-derived lymphocytes (B cells) produce antibodies which can react specifically with antigen. Because B cells produce antibodies that circulate and the immunity is called humoral immunity. Antigen specific antibodies produced in the humoral immune response can be used in immunological assays to detect various disease agents or antigenic molecules. With the development of monoclonal antibodies, greater specificity and reproducibility can be obtained with immunological assays. The remainder of this discussion will be devoted to the humoral immune response. The Humoral Immune Response