Lymphocytes are white blood cells that defend the body against infection by recognizing foreign material and producing chemicals to destroy it. B and T lymphocytes are specific to one type of antigen, and T cells have two main types: killer T cells and T helper cells. The primary immune response is slow, but memory cells provide immunity for years and allow for a rapid secondary immune response to prevent symptoms.
Lymphocytes are a type of white blood cell, which are an important part of the immune system. Lymphocytes can defend the body against infection because they can distinguish body cells from foreign ones. Once they recognize 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 within the bone marrow until they are mature. Once mature, they spread throughout the body and concentrate in the spleen and lymph nodes. T lymphocytes, or T cells, leave the bone marrow and mature in the thymus, a gland found in the chest. Only mature lymphocytes can carry out immune responses.
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, whether B or T, is specific for only one type of antigen. Only when the appropriate antigen is encountered 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 cells infected with antigens. When a killer T cell recognizes an antigen attached to a cell in the body, it attaches itself to the surface of the infected cell. It then secretes toxic chemicals into the cell, killing both the antigen and the infected cell.
T helper cells release a chemical, called a cytokine, when activated by an antigen. These chemicals then stimulate the B lymphocytes to initiate their immune response. When a B cell is activated, it produces antigen-fighting proteins, called antibodies. Antibodies are specific to 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.
Plasma cells continue to multiply and produce antibodies during infection, but they don’t live very long. Plasma cells die within a few days. The antibodies stay in the system a little longer, but usually break down within a week. Memory cells stay in the body much longer than plasma cells and antibodies, often years. They are important for providing immunity.
If the antigen infects the body again, the memory cells respond almost immediately. They immediately begin to multiply 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 prevented. This is known as a secondary immune response and is what gives people immunity to a disease.
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