Lymphocyte: Definition, Types, Functions, Counting, Killer Cells and Memory Cells

Leukocyte of small size and rounded nucleus which is normally present in lymphatic tissues and blood.

Lymphocyte, type of white blood cell (leukocyte) that is of fundamental importance in the immune system because lymphocytes are the cells that determine the specificity of the immune response to infectious microorganisms and other foreign substances.

In human adults, lymphocytes account for approximately 20 to 40 percent of the total number of white blood cells.

They are found in the circulation and are also concentrated in central lymphoid organs and tissues, such as the spleen, tonsils and lymph nodes, where the initial immune response is likely to occur .

Types and functions

The two main types of lymphocytes are B lymphocytes and T lymphocytes, or B cells and T cells. Both originate from stem cells in the bone marrow and are initially similar in appearance.

Some lymphocytes migrate to the thymus, where they mature into T cells; others remain in the bone marrow, where, in humans, they develop into B cells.

Most lymphocytes are of short duration, with an average life of one week to a few months, but some live for years, providing a set of long-lived T and B cells. These cells represent the immunological “memory”, a faster and more vigorous response to a second encounter with the same antigen.

Through receptor molecules on their surfaces, lymphocytes can bind antigens (foreign substances or microorganisms that the host recognizes as “non-self”) and help eliminate them from the body.

Each lymphocyte has receptors that bind to a specific antigen. The ability to respond to virtually any antigen comes from the huge variety of lymphocyte populations that the body contains, each with a receptor capable of recognizing a single antigen.

Once stimulated by binding to a foreign antigen, such as a component of a bacterium or virus, a lymphocyte multiplies in a clone of identical cells. Some of the cloned B cells differentiate into plasma cells that produce antibody molecules.

These antibodies are tightly modeled after the precursor B cell receptors and, once released into the blood and lymph, bind to the target antigen and initiate its neutralization or destruction. The production of antibodies continues for several days or months, until the antigen is exceeded.

Other B cells, memory B cells, are stimulated to multiply but do not differentiate into plasma cells; They provide the immune system with a lasting memory.

In the thymus, the T cells multiply and differentiate into T helper, regulatory or cytotoxic cells or become T cells of memory. They are then seeded into the peripheral tissues or circulate in the blood or lymphatic system.

Once stimulated by the appropriate antigen, the T helper cells secrete chemical messengers called cytokines, which stimulate the differentiation of B cells into plasma cells, thus promoting the production of antibodies.

Regulatory T cells act to control immune reactions, hence their name. Cytotoxic T cells, which are activated by various cytokines, bind and destroy infected cells and cancer cells.

B cells:

B cells develop from bone marrow stem cells in adults. When B cells are activated due to the presence of a particular antigen, they create antibodies that are specific for that specific antigen.

Antibodies are specialized proteins that travel through the bloodstream and are found in body fluids.

Antibodies are essential for humoral immunity since this type of immunity is based on the circulation of antibodies in body fluids and blood serum to identify and counteract antigens.

T cells:

T cells develop from stem cells of the liver or bone marrow that mature in the thymus. These cells play an important role in cell-mediated immunity.

T cells contain proteins called T cell receptors that populate the cell membrane. These receptors are capable of recognizing several types of antigens.

There are three major classes of T cells that play specific roles in the destruction of antigens. They are cytotoxic T cells, helper T cells and regulatory T cells.

Cytotoxic T cells directly terminate cells containing antigens by binding to them and lysing or causing them to break.

Auxiliary T cells precipitate antibody production by B cells and also produce substances that activate other T cells.

Regulatory T cells (also called suppressor T cells) suppress the response of B cells and other T cells to antigens.

Lymphocyte counts

Lymphocytes are a component of the complete blood count (CBC) tests that include a differential of white blood cells, in which the levels of the major types of white blood cells are measured.

These tests are used to help in the detection, diagnosis and control of various medical conditions.

Lymphocyte counts that are below the reference range, which varies for adults and children, may be indicative of lymphocytopenia ( lymphopenia ), while those above are a sign of lymphocytosis.

Lymphocytopenia is associated with a variety of conditions, ranging from malnutrition to rare hereditary disorders, such as ataxia- telangiectasia or severe combined immunodeficiency syndrome.

Lymphocytosis is usually associated with infections, such as mononucleosis or whooping cough , certain cancers of the blood or lymphatic system, such as multiple myeloma and chronic lymphocytic leukemia, and autoimmune disorders that cause chronic inflammation, such as inflammatory bowel disease .

Células Natural Killer (NK)

Natural killer cells function similarly to cytotoxic T cells, but they are not T cells. Unlike T cells, the response of NK cells to an antigen is nonspecific.

They do not have T cell receptors or trigger antibody production, but they are able to distinguish infected or cancerous cells from normal cells.

NK cells travel through the body and can bind to any cell they come in contact with. The receptors on the surface of the natural killer cell interact with the proteins in the captured cell. If a cell activates more activating receptors of the NK cell, the death mechanism will activate.

If the cell activates more inhibitory receptors, the NK cell will identify it as normal and leave the cell alone. NK cells contain granules with chemicals that, when released, break down the cell membrane of diseased or tumor cells.

This eventually causes the target cell to explode. NK cells can also induce infected cells to undergo apoptosis (programmed cell death).

Memory cells

During the initial course of response to antigens such as bacteria and viruses, some T and B lymphocytes become cells known as memory cells. These cells allow the immune system to recognize the antigens that the body has previously found.

Memory cells direct a secondary immune response in which antibodies and immune cells, such as cytotoxic T cells, occur more rapidly and for a longer period of time than during the primary response.

The memory cells are stored in the lymph nodes and the spleen and can remain during the life of an individual. If enough memory cells are produced when an infection is found, these cells can provide lifelong immunity against certain diseases such as mumps and measles.