Lymphocyte
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A lymphocyte is a type of white blood cell involved in 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 defenses.
T cells are chiefly responsible for cell-mediated immunity whereas B cells are primarily responsible for humoral immunity (relating to antibodies). T cells are named such because these lymphocytes mature in the thymus; B cells, named for the bursa of Fabricius in which they mature in bird species, are thought to mature in the bone marrow in humans.
In the presence of an antigen, B cells can become much more metabolically active and differentiate into plasma cells, which secrete large quantities of antibodies.
Microscopically, in a Wright's stained peripheral blood smear, a normal lymphocyte has a large, dark-staining nucleus with little to no basophilic cytoplasm. In normal situations, the coarse, dense nucleus of a lymphocyte is approximately the size of a red blood cell (about 7 micrometres 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.
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. When one must specifically determine the percentage of lymphocytes that produce a particular secretion (say, a specific antibody or cytokine), the ELISPOT or secretion assay techniques can be used instead.
The human immunodeficiency virus (HIV) hijacks and destroys T cells (specifically, CD4+ lymphocytes). Without this key defense, the 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 (in some rare cases, 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.
The formation of lymphocytes is known as lymphopoiesis.
See also
Granulocytes Normal blood contains from 5000 to 9000 White Blood Cells (WBC’s) per cubic millimeter and 70 to 75% of them are granulocytes. These cells are formed and mature in the bone marrow and are continually being released into the blood. They make up 20 to 25% of the WBC’s. The granulocytes have multilobular nuclei and are 9 to 12 μm in diameter. (Out of the Texas Woman's University Microbiology Text Book.) [link title]]
- Agranulocyte
- Anergy
- Complete blood count
- Cytotoxicity
- Human leukocyte antigen
- Lymphoproliferative disorders
- Reactive lymphocyte
- Secretion assay
| Lymphatic system - [http://encycl.opentopia.com/ edit] |
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| Lymph nodes > Lymph | Lymphocytes | Lymph vessels | Thoracic duct | Immune system | Bone marrow | Spleen | Thymus | Tonsils |
| Blood - Blood plasma - [http://encycl.opentopia.com/ edit] |
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| Pluripotential hemopoietic stem cells | Red blood cells (Reticulocyte, Normoblast) | White blood cells |
| Lymphocytes (Lymphoblast) |
| T cells (Cytotoxic, Helper, Regulatory T cells, Natural Killer T cells) | B cells (Plasma cells & Memory B cells) | Natural killer cells |
| Myelocytes (Myeloblast) |
| Granulocytes (Neutrophil granulocyte>Neutrophil, Eosinophil, Basophil) | Mast cell precursors | Monocytes (Histiocyte, Macrophages, Dendritic cells, Langerhans cells, Microglia, Kupffer cells, Osteoclasts) | Megakaryoblast | Megakaryocyte | Platelets |
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