Lymphocytes

Figure19. EM of a medium-sized lymphocyte; M, mitochondria; G, Golgi apparatus; C, centriole

Lymphocytes are the most common agranulocytes and account for about 30% of the total blood leucocytes. Lymphocytes are the main functional cells of the lymphatic or immune system.

Three groups of lymphocytes can be identified according to size: small, medium and large, ranging in diameter from 6 to 18 μm. In the bloodstream, most lymphocytes are small or medium in size, 6-12 μm.

In understanding the function of the lymphocytes, it is important to realize that the majority of the lymphocytes found in blood or lymph represent recirculating immunocompetent cells, i.e., cells that have developed the capacity to recognize and respond to foreign antigen and are in transit from one site of lymphatic tissue to another.

In the body presents two functionally distinct types of lymphocytes; T lymphocytes and B lymphocytes. In human blood, it has been found approximately 90% of the lymphocytes are mature T cell, and 4-10% are mature B cells. Approximately 5% of the cells are identified as null lymphocytes. These cells may be circulating stem cells.

T lymphocytes are T cells, thymus-dependent lymphocytes. T lymphocytes have a long life span, and are involved in cell-mediated immunity.

B lymphocytes have variable life spans and are involved in the humoral immunity in the production of the several varieties of circulating antibodies.

B lymphocytes are B cells, so named because they were first recognized as a separate population in the bursa of Fabricius in birds.

Lymphocytes differentiate into effector cells after first encounter a specific antigen.

For example;

-B lymphocytes may divide several times, producing more B lymphocytes and large clones of cells that differentiate into plasma cells involved in the production of antibodies.

-T lymphocytes may undergo several rounds of cell division, producing cells that differentiate into cytotoxic T lymphocytes (killer T lymphocytes, helper T lymphocytes or suppressor T lymphocytes).

-some B and T cells do not undergo differentiation into effector cells but serve as long lived memory cells, circulating lymphocytes that are primed to respond more rapidly.

Main functions are:

-lymphocytes are the key cells in the immune system. They recognize foreign antigen and react immunologically to them.

Дата добавления: 2015-07-25