What are lymphocytes? (with photos)

Two types of lymphocytes are produced in the bone marrow before birth.

Lymphocytes are a type of white blood cell, which is an important part of the immune system. Lymphocytes can defend the body against infection because they can distinguish the body’s own cells from foreign cells. After recognizing the foreign material in the body, they produce chemicals to destroy that material.

Two types of lymphocytes are produced in the bone marrow before birth. B lymphocytes, also called B cells, remain in the bone marrow until they are mature. Once mature, they spread throughout the body and are concentrated in the spleen and lymph nodes. T lymphocytes, or T cells, leave the bone marrow and mature in the thymus, a gland located in the chest. Only mature lymphocytes can carry out immune responses.

A diagram showing different types of white blood cells, including lymphocytes.

All lymphocytes are capable of producing chemicals to fight off foreign molecules. Any molecule recognized by the body as foreign is called an antigen. A lymphocyte, either B or T, is specific for only one type of antigen. Only when the appropriate antigen is found is the cell stimulated.

There are two main types of T lymphocytes and each plays a separate role in the immune system. Killer T cells search the body for infected cells for antigens. When a killer T cell recognizes an antigen bound to a cell in the body, it binds to the surface of the infected cell. It then secretes toxic chemicals into the cell, killing both the antigen and the infected cell.

Lymphocytes are found in the blood and help defend the body against infections.

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Helper T cells release a chemical called a cytokine when activated by an antigen. These chemicals stimulate B lymphocytes to start their immune response. When a B cell is activated, it produces proteins that fight antigens, called antibodies. Antibodies are specific for only one antigen, so there are many types of B cells in the body.

The first time an antigen is encountered, the primary immune response, the reaction is slow. After being stimulated by helper T cells, B cells begin to replicate and become plasma cells or memory cells. Plasma cells produce antibodies to fight the antigen, but the antigen also has time to multiply. The effect of the antigen on the body’s cells is what causes the symptoms of the disease. Initially, it can take days or even weeks for enough antibodies to be produced to defeat the invading material.

Tuberculosis can cause a low lymphocyte count.

Plasma cells continue to multiply and produce antibodies during infection, but they do not live long. Plasma cells die within a few days. Antibodies stay in the system for a little longer, but usually break down within a week. Memory cells remain in the body much longer than plasma cells and antibodies, often years. They are important to provide immunity.

Each antibody is specific for a particular antigen.

If the antigen infects the body again, the memory cells respond almost immediately. They begin to multiply immediately and become plasma cells. This causes antibodies to be produced virtually instantly. In these later infections, the response is so rapid that symptoms can be avoided. This is known as a secondary immune response and is what gives people immunity to a disease.

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