Lymphocytes are the second most numerous type of leucocyte, constituting approximately 30% of circulating white cells. There are two types of lymphocytes, B and T cells, both of which derive from bone marrow stem cells. They migrate from peripheral circulation to the spleen and lymph nodes and then return back to the blood stream. B cells accumulate in the lymph node and spleen follicles and the cortex periphery and medullary cords of the lymph nodes. T cells are found in the paracortical areas of the lymph nodes and periarteriolar sheaths surrounding the spleen arterioles.
Lymphocytes are differentiated from other leucocytes by Side Fluorescent Light and Forward Scattered Light measurements in the Sysmex XN WDF Channel.
The majority of small lymphocytes have scanty pale blue cytoplasm and are roughly circular in shape. There are about 10% of larger lymphocytes in peripheral blood with more abundant cytoplasm and less condended nuclear chromatin.
Lymphocyte Function
Lymphocytes are immunologically competent cells which both assist phagocytic leucocytes and produce immunoglobulins to fight infection directly. B cells produce surface immunoglobulins which are specific for foreign antigens. These cells retain immunological memory, remaining in circulation for many years. When triggered by the particular antigen they will proliferate and become plasma cells containing intracellualr immunoglobulins for secretion.
T cells also have surface receptors for antigens but do not secrete immunoglobulins. The helper T cells (CD4+) assist antigen presenting B cells by producing signals to induce further B and T cells to proliferate. Killer T cells (CD8+) are directly cytoxic to foreign micro-organisms. Natural killer cells are also CD8 positive but lack the T-cell receptor. They are able to kill cells which do not display class 1 HLA molecules on their surface such as virally infected or malignant cells. They are also able to kill abnormally produced lymphocytes which fail to express these molecules.
Causes of a raised lymphocyte count (lymphocytosis):
Physiologic - exercise,seizure
Infection - Bacterial (pertussis, brucellosis, Tb, syphilis), Viral (rubella, mumps, varicella, infectious mononucleosis, hepatitis, herpes, HIV)
Haematologic - Lymphoid leukaemias/lymphomas
Drug reactions
Splenomegaly - splenic sequestration
Addison's disease
Thyrotoxicosis
Causes of a lowered lymphocyte count (lymphopenia):
Bone marrow failure - malignancy, irradiation
Infection - HIV, Legionella
Post chemotherapy/radiotherapy
Steroid therapy