Lymphocyte
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 the large granular lymphocytes and the small lymphocytes. The large granular lymphocytes are more commonly known as the
natural killer cells (NK cells). The small lymphocytes are the
T cells and
B cells. Lymphocytes play an important and integral part of the body's defenses.
The NK cells are a part of the
cell-mediated immunity. They can attack the host's cells that express a foreign peptide on their
MHC class I surface proteins. Furthermore, the NK cells do not require activation in order to perform their
cytotoxicity upon target cells.
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.
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.)
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Agranulocyte*
Anergy*
Complete blood count*
Cytotoxicity*
Human leukocyte antigen*
Lymphoproliferative disorders*
Reactive lymphocyte*
Secretion assay
