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A lymphocyte is one of the subtypes of a white blood cell in a vertebrate's immune system.

Lymphocytes include natural killer cells , T cells , and B cells .

They are the main type of cell found in lymph, which prompted the name "lymphocyte".

The three major types of lymphocyte are T cells, B cells and natural killer cells.

Lymphocytes can be identified by their large nucleus.

T cells and B cells are the major cellular components of the adaptive immune response.

T cells are involved in cell-mediated immunity, whereas B cells are primarily responsible for humoral immunity .

The function of T cells and B cells is to recognize specific "non-self" antigens, during a process known as antigen presentation.

Once they have identified an invader, the cells generate specific responses that are tailored maximally to eliminate specific pathogens or pathogen-infected cells.

B cells respond to pathogens by producing large quantities of antibodies which then neutralize foreign objects like bacteria and viruses.

In response to pathogens some T cells, called T helper cells, produce cytokines that direct the immune response, while other T cells, called cytotoxic T cells, produce toxic granules that contain powerful enzymes which induce the death of pathogen-infected cells.

Following activation, B cells and T cells leave a lasting legacy of the antigens they have encountered, in the form of memory cells.

Throughout the lifetime of an animal, these memory cells will "remember" each specific pathogen encountered, and are able to mount a strong and rapid response if the same pathogen is detected again; this is known as acquired immunity.

NK cells are a part of the innate immune system and play a major role in defending the host from tumors and virally infected cells.

NK cells distinguish infected cells and tumors from normal and uninfected cells by recognizing changes of a surface molecule called MHC class I. NK cells are activated in response to a family of cytokines called interferons.

Activated NK cells release cytotoxic granules which then destroy the altered cells.

They are named "natural killer cells" because they do not require prior activation in order to kill cells which are missing MHC class I. Mammalian stem cells differentiate into several kinds of blood cell within the bone marrow.

This process is called haematopoiesis.

All lymphocytes originate, during this process, from a common lymphoid progenitor before differentiating into their distinct lymphocyte types.

The differentiation of lymphocytes follows various pathways in a hierarchical fashion as well as in a more plastic fashion.

The formation of lymphocytes is known as lymphopoiesis.

B cells mature into B lymphocytes in the bursa equivalent, which in humans is the GALT, which is thought to be located in the Peyer's patches of the intestine, while T cells migrate to and mature in a distinct organ, called the thymus.

Following maturation, the lymphocytes enter the circulation and peripheral lymphoid organs where they survey for invading pathogens and/or tumor cells.

The lymphocytes involved in adaptive immunity differentiate further after exposure to an antigen; they form effector and memory lymphocytes.

Effector lymphocytes function to eliminate the antigen, either by releasing antibodies , cytotoxic granules or by signaling to other cells of the immune system .

Memory T cells remain in the peripheral tissues and circulation for an extended time ready to respond to the same antigen upon future exposure; they live weeks to several years, which is very long compared to other leukocytes.

Microscopically, in a Wright's stained peripheral blood smear, a normal lymphocyte has a large, dark-staining nucleus with little to no eosinophilic cytoplasm.

In normal situations, the coarse, dense nucleus of a lymphocyte is approximately the size of a red blood cell .

Some lymphocytes show a clear perinuclear zone around the nucleus or could exhibit a small clear zone to one side of the nucleus.

Polyribosomes are a prominent feature in the lymphocytes and can be viewed with an electron microscope.

The ribosomes are involved in protein synthesis, allowing the generation of large quantities of cytokines and immunoglobulins by these cells.

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.

This can be used to determine the percentage of lymphocytes that contain a particular combination of specific cell surface proteins, such as immunoglobulins or cluster of differentiation markers or that produce particular proteins .

In order to study the function of a lymphocyte by virtue of the proteins it generates, other scientific techniques like the ELISPOT or secretion assay techniques can be used.