Lymphocytes are a type of white blood cell (or leukocyte). They help an organism to fight infections. They occur in the immune system of all vertebrates.
In human adults lymphocytes make up roughly 20 to 40 percent of the total number of white blood cells. They are found in the circulation and also are concentrated in central lymphoid organs and tissues, such as the spleen, tonsils, and lymph nodes, where the initial immune response is likely to occur.
Image: Scanning electron micrograph of a human T lymphocyte (also called a T cell) from the immune system of a healthy donor.
Lymphocytes can be found
- In the veins and arteries (in the body’s circulation).
- In the lymph nodes and lymph channels of the body’s lymphatic system.
- Scattered all over the body eg in the spleen, tonsils, intestines, and in the lining of the airways. Here the lymphocytes represent what is referred to as “lymphoid tissue.”
- In the intestines, in a stretch of territory called Peyer’s Patches. The lymphocytes here are more highly organized, in structures called follicles (forming an important part of the immune system by monitoring bacteria that live in the gut and preventing the growth of bad bacteria in the intestines).
There are three main types of lymphocytes: B cells, T cells, and natural killer cells. Two of these types of lymphocytes are critical for specific immune responses. They are B lymphocytes (B cells) and T lymphocytes (T cells).
- T and B cells 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 short-lived, with an average life span of a week to a few months, but a few live for years, providing a pool of long-lived T and B cells. These cells account for immunologic “memory,” a more rapid, vigorous response to a second encounter with the same antigen.
- T cells (thymus cells) and B cells (bone cells) are the main cells of the adaptive immune response.
T cell lymphocytes are different from B cells and natural killer cells in that they have a protein called a T-cell receptor on their cell membrane. T-cell receptors are capable of recognizing various types of specific antigens (substances that provoke an immune response). Unlike B cells, T cells do not utilize antibodies to fight germs.
There are several types of T cell lymphocytes, each with specific functions in the immune system. Common T cell types include:
- Cytotoxic T cells (also called CD8+ T cells) - are involved in the direct destruction of cells that have become cancerous or are infected by a pathogen. Cytotoxic T cells contain granules (sacs containing digestive enzymes or other chemical substances) that they utilize to cause the target cell to burst open in a process called apoptosis.
- Helper T cells (also called CD4+ T cells) - precipitate the production of antibodies by B cells and also produce cytokines that activate cytotoxic T cells and white blood cells known as macrophages. CD4+ cells are targeted by HIV. HIV infects helper T cells and destroys them by triggering signals that result in T cell death.
- Regulatory T cells (also called suppressor T cells) - suppress the response of B cells and other T cells to antigens. This suppression is needed so that an immune response does not continue once it is no longer needed.
- Memory T cells - help the immune system to recognize previously encountered antigens and respond to them more quickly and for a longer period of time. Helper T cells and cytotoxic T cells can become memory T cells. Memory T cells are stored in the lymph nodes and spleen and may provide lifetime protection against a specific antigen in some cases.
An important difference between T-cells and B-cells: B-cells connect to antigens right on the surface of the invading virus or bacteria; T-cells, connect to virus antigens on the outside of infected cells.
Our body has up to 10 billion different B-cells. Image: B cell activation naive to plasma cell.
B-cells become plasma cells
- When a B-cell receptor connects to its specific antigen, a Helper T-cell releases chemicals that tell that B-cell to divide many times. This makes an army of B-cells with the perfectly shaped B-cell receptor to connect to the invader ie the antigen.
- Many of these B-cells quickly turn into plasma cells. Plasma cells make and release antibodies that connect to the same antigen as the original B-cell receptor. Plasma cells make thousands of antibodies per second, which spread throughout your body, trapping any viruses they see along the way.
- Unactivated B cells circulate in the blood until they come in contact with an antigen and become activated.
- Once activated, B cells produce the antibodies needed to fight against infection.
B cells are necessary for adaptive or specific immunity, which focuses on the destruction of foreign invaders that have invaded the bodies initial defenses. Adaptive immune responses are highly specific and provide long-lasting protection against the pathogens that elicit the response.
Natural killer cells
Natural killer (NK) cells are a part of the immune system a person is born with. They protect the organism from tumours and from cells infected by viruses. Natural killer cells are made active by chemicals released by damaged cells called interferons.
Natural killer cells can tell the difference between infected cells, tumours, and normal cells. They recognise changes in the level of a surface molecule called MHC (major histocompatibility complex) class I.
The NK cells then release cytotoxic (cell-killing) granules which then destroy the altered cells.
When white blood cells are counted, the lymphocyte count is the percentage of lymphocytes present.
- An increase in lymphocyte concentration is usually a sign of a viral infection (in some rare case, leukemias are found through an abnormally high lymphocyte count in an otherwise normal person).
- A low normal to low absolute lymphocyte count is found with infections after surgery or trauma.
- Human immunodeficiency virus (HIV) infects and destroys T cells. Without the key defence that these T cells provide, the body becomes prone to infections that otherwise would not affect healthy people.
- The extent of HIV progression is found by measuring the percentage of CD4+ T cells in the patient's blood. The effects of other virus or lymphocyte disorders can also often be estimated by counting the numbers of lymphocytes present in the blood.
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