What are the Cells of Lymphoid Lineage?

Production of lymphocytes from pleuripotent hemopoietic stem cells is known as lymphopoiesis. Lymphocytes are the chief constituents of the immune system.

The oligopotent (In biology, oligopotency is the ability of progenitor cells to differentiate into a few cell types) lymphoid progenitors derived from pleuripotent haemopoitic stem cells generate 3 types of cells through an orderly change; they are T Lymphocytes, B Lymphocytes and Null cells.

Local hormones or cytokines influence the differentiation of lymphoid stem cells into different lymphocytes. Even though all the lymphocytes are derived from the same oligopotent lymphoid stem cells, their function and the surface receptors /markers differ.

The micro environment of lymphoid stem cells influences the differentiation of stem cells. Hence the T and B Lymphocytes appearing morphologically similar differ in their functional characteristics and surface markers.

Since both T and B lymphocytes are morphologically similar, it is impossible to distinguish T cells and B cells in a peripheral blood smear. The surface receptors present on the lymphocytes confer specificity for antigen and immune response.

The affinity of receptors for the different antigens varies. As the lymphocytes with the highest affinity for the most abundant antigens help in defense, their progeny are preferably generated. This kind of multiplication of specific lymphocytes is known as clonal expansion. Unlike T and B cells, null cells do not show any specific surface markers.

A normal lymphocyte shows large, darkly stained nucleus with little to no basophilic cytoplasm. In normal situations, the coarse, dense nucleus of a lymphocyte is approximately about 7 micrometers in diameter.

Some lymphocytes show a clear perinuclear zone (or halo) around the nucleus or could exhibit a small clear zone to one side of the nucleus. Presence of polyribosome is a prominent feature in the lymphocytes.

The lymphocytes following exposure to an antigen differentiate further to effector and memory lymphocytes. Function of effector lymphocytes is elimination of antigens, either by releasing antibodies (in the case of B cells), cytotoxic granules (in the case of cytotoxic T cells) or by signalling to other cells of the immune system (helper T cells). Memory cells remain in the peripheral tissues and circulation to respond readily to the same antigen upon future exposure.