Lymphocyte

Lymphocytes are white blood cells, part of the human body's immune system. There are two broad categories of lymphocytes, namely T cells and B cells. Lymphocytes play an important and integral part of the body's defense.

T-cells are responsible for cell mediated immunity whereas B-cells are responsible for humoral immunity (relating to antibodies). T-cells are named such because these lymphocytes mature in the Thymus and B-cells mature in bone marrow.

In the presence of an antigen, B-cells become much more metabolically active and transform into Plasma cells. Plasma cells are large lymphocytes with a large nucear to cytoplasmic ratio and are the form of B-cell lymphocytes that produce antibodies.

Microscopically, in Wright's stained peripheral blood smear, a normal lymphocyte has a large, dark-staining nucleus with little to no basophyllic cytoplasm. In normal situations, the nucleus of a lymphocyte is approximately the same size as a red blood cell (about 7 micrometres in diameter). It is impossible to distinguish between T-cells and B-cells in a peripheral blood smear. Normally, flow cytometry testing is used for specific lymphocyte population counts.

The Human Immunodeficiency Virus hijacks and destroys T-cell lymphocytes (specifically, CD4 lymphocytes). Without this key defense, your body is susceptible to opportunistic diseases that otherwise would not kill healthy people.

A lymphocyte count is part of a peripheral complete blood cell count and is expressed as percentage of lymphocytes to total white blood cells counted. An increase in lymphocytes is usually a sign of a viral infection (rarely, leukemias are found through an abnormally raised lymphocyte count in an otherwise normal person). A general increase in the number of lymphocytes is known as lymphocytosis whereas a decrease is lymphocytopenia.

Immune system - Lymphatic system
Lymph nodes - Lymph - Lymphocytes - Lymph vessels - Thoracic duct
Bone marrow - Spleen - Thymus - Tonsils