T- and B-lymphocytes can be distinguished by specific laboratory methods and not by morphology alone. They have different roles in generating acquired immunity: T-lymphocytes in cellular, and B-lymphocytes in humoral immunity. T-lymphocytes help B-lymphocytes to produce antibodies and B-lymphocytes can control T-lymphocytes activity.
T- and B-cells cooperate both mutually and with macrophages and granulocytes. Together with natural killer-cells (NK-cells) and the complement system they compose a powerful defensive system in the organism.

Terminally differentiated B-cells, plasma-cells, secrete immunoglobulins. B-lymphocytes have immunoglobulins bound to their membranes, most often IgM; as well as receptors for complement and for Fc fragment of certain immunoglobulins. T-lymphocytes have no such receptors and are unable to bind immunoglobulins.
T-lymphocytes are responsible for cellular immunity, reactions of delayed sensitivity, rejection of transplant organs and tissues, and for some other immunologic reactions in the organism. T-lymphocytes also influence the amount and quality of humoral immune response.
If they get in touch with antigen with which they have not been in touch, or with antigen with which they met before and whose features they remember (“memory cells”), both B- and T-lymphocytes undergo to lymphoblastic transformation, blastogenesis. This results in the production of antibodies by B-lymphocytes or plasma cells, or the immunity reaction bound to the cells (T-lymphocytes).
T-lymphocytes are more active. They continually circulate between lymphoid organs and blood via thoracic duct and have long life: about 4.4 years, and some over 20 years. B-lymphocytes are very immobile, and live shortly: mostly only few days. Therefore, their number in the peripheral blood is unequal: 12-34% of B-lymphocytes, and 66-88% of T-lymphocytes.