 Lymphocytes are responsible for the specificity and memory in adaptive immune responses.  They are produced in the primary lymphoid organs and function in the secondary lymphoid organs / tissues where they recognize and respond to foreign antigens.  There are three types of lymphocytes – NK cells, T cells and B cells, although only T and B cells have true antigen specifiicity and memory.  T cells and B cells mature in the thymus and bone marrow, respectively. In the resting state both T and b lymphocytes have a similar morphology with a small amount of cytoplasm.

 They have specific but different antigen receptors and a variety of other surface molecules necessary for interaction with other cells.  These include molecules required for their activation and for movement into and out of the tissues of the body.  This ability to migrate into the tissues and return via the lymphatic vessels to the blood stream is a unique feature of lymphocytes.  There are two classes of T lymphocytes, T helper cells and T cytotoxic cells. All T lymphocytes have antigen receptors (TcR) which provide their specificity and CD3 which is essential for their activation.  These molecules also serve as “Markers” to identity T cells.  B lymphocytes make and use antibodies are their specific antigen receptor. They have molecules similar to CD3, i.e. CD79 which are important in their activation.  B lymphocytes can mature into plasma cells which produce and secrete large amounts of antibody.

T cellsB cells Site of maturationThymusBone marrow Antigen receptorTcRAntibody Requirement of MHC for recognition YesNo Characteristic ‘markers’All have TcR/ CD3 Th – CD4 Th – CD8 Surface Ig. CD19/CD20/CD21 CD79 Main location in Lymph nodes Paracortical areaFollicles Memory cellsYes FunctionProtects against intracellular microbes Protects against extra- cellular microbes ProductsTh1 – IFN / TNF α Th2 – IL4, IL5,IL6 Tc – perforins Antibodies (B cells mature into plasma cells)

 B cells are produced in the bone marrow and like T cells migrate to the secondary lymphoid organs and tissues where they responed to foreign antigens. There are two kinds of B cells.  B1 cells: These cells arise early in ontogeny, express mainly lgM antibodies encoded by germ-line antibody genes, mature independently of the bone-marrow, generally recognize multimmeric sugar / lipid antigens of microbes and are T cell independent.  B2 cells: These cells are the B cells primarily responsible for the development of humoral mediated immunity. They are produced in the bone marrow, and with the help of T cells produce lgG, lgA and lgE antibodies.

 B cells are mainly found in loose aggregates in lymphoid tissues or in well-defined proliferating foci.  Antibodies are the antigen receptor on these cells and are associated with several other molecules which form the B cells receptor complex.  When activated by antigen and, in most cases, with T cell help, B cells proliferate and mature into memory cells or plasma cells.  Memory cells only produce antibody for expression on their cell surface and remain able to respond to antigen if it is reintroduced.  In contrast, plasma cells do not have cell surface antibody receptors.  Rather these cells, which are derived from activated B cells, function as factories producing and secreting large amounts of antibody of the same specificity as the antigen receptor on the stimulated parent B cells.

THANK YOU