Lymphocytes are found in large numbers in blood, lymph (the clear fluid of the lymphatic vessels) and in lymphoid organs such as the thymus, lymph nodes and spleen. They are essential for immunity. B-lymphocytes produce antibody against a specific antigen (humoral immunity) while T-lymphocytes are the cells of the cell-mediated response. Primary lymphoid organs (bone marrow, thymus) are the sites of lymphoid development. In the secondary lymphoid organs (lymph nodes, spleen), mature lymphocytes meet antigens and the immune response is triggered.

Most mature lymphocytes appear under the light microscope as cells with round nuclei and a thin rim of agranular cytoplasm (Fig 4.1). Although B- and T-cells are not distinguishable by their morphology, there are major differences in their mode of maturation and function.


T-cells make up 75% of the lymphocytes of the blood and form the basis of cell-mediated immunity. They are less autonomous than their B-cell companions, needing the cooperation of antigen-presenting cells expressing self-histocompatibility molecules (human leucocyte antigens (HLA)) for the recognition of the antigen by the T-cell receptor (TCR) (Fig 4.2).

T-cells originate in the marrow but many are destroyed in subsequent processing by the thymus, the objective being to select the minority of cells which will recognise self-HLA but not react with self-tissue antigens. The maturation sequence is characterised by changing patterns of cell surface molecules (Fig 4.3). Mature T-cells are divisible into two basic types. About two-thirds of blood T-cells are ‘helper’ cells expressing the surface marker CD4, while the remainder express CD8 and are mostly of ‘cytotoxic’ type.

It appears that helper cells recognise the combination of antigen and self-HLA class II molecules on the antigen-presenting cell, and cytotoxic cells bind with antigen in conjunction with HLA class I molecules on the target cell (Fig 4.2

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Jun 12, 2016 | Posted by in HEMATOLOGY | Comments Off on Lymphocytes

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