a variety of white blood cells (agranular leukocytes) in vertebrates and man.

Lymphocytes are spherical in shape and contain an oval nucleus, surrounded by ribosome-rich cytoplasm. Lymphocytes are distinguished, by size, as small (diameter, 4.5–6.5 microns [μ]), large (10–18 μ), and medium (6.5–10 μ). Cells of the small type constitute 95 percent of the total number of lymphocytes. Unlike lymphocytes of the large and medium types, these are incapable of mitosis. However, under the influence of phytohemagglutinin, they turn into large and medium lymphocytes, enter the mitotic cycle, and divide (this method is used in medical genetics for analyzing the cells’ chromosome apparatus). The majority of small lymphocytes are long-lived (in cases, surviving the life of the individual), circulating repeatedly between the lymph and the blood. The short-lived forms survive three to six days.

In mammals and man, lymphocytes are formed in the thymus gland, lymph nodes, spleen, bone marrow (short-lived only), and lymphoid-tissue aggregates (mainly along the digestive tract). The original form from which the lymphocytes of adult mammals develop is the hematopoietic stem cell, the daughter cells of which at first multiply in the thymus and then settle and multiply in the lymph nodes.

The principal functions of the lymphocytes are hematopoietic (certain lymphocytes presumably being capable of differentiation into other blood and connective-tissue cells), trophocytic (possibly serving as suppliers of nutrients and building materials to other cells), and immunological.

The destruction of lymphocytes (by ionizing radiation, antilymphocytic serum, immunodepressant medications) suppresses the body’s immunological reactivity. Advantage is taken of this in cases of tissue and organ transplant (to prevent rejection) and in the treatment of a number of diseases.