A lymphocyte is a type of white blood cell (WBC) that has a significant function in the body's immune system. Lymphocytes function by properly responding to infectious microorganisms and other foreign invaders in the body. Lymphocytes can work alone, but some of them are also capable of coordinating with other cells.
Approximately 20 to 40 percent of the total number of white blood cells are lymphocytes. They are usually found in the blood and are concentrated in the tonsils, lymph nodes, and spleen, where initial immune responses are likely to take place.
The two main types of lymphocytes are B cells (B lymphocytes) and T cells (T lymphocytes). These two lymphocytes originate from the stem cells found in the bone marrow and initially look similar in their appearance. Some of the lymphocytes move to the thymus and mature into T cells, while other lymphocytes stay in the bone marrow and develop into B cells.
Usually, lymphocytes only last for a short time, having an average lifespan of seven days to a few months. However, a few lymphocytes are able to survive for years and create a pool of T and B cells. These cells account for the immunological memory or a faster immunologic response to previously encountered antigens.
Lymphocytes are also able to bind antigens through the receptor molecules found on their surfaces. Antigens are foreign microorganisms or substances that the host organism recognizes as non-self. Every lymphocyte has receptors that bind to specific antigens.
Different Types of Lymphocytes
1. T cells
T cells mature in the thymus but originate from the bone marrow. There are different types of T cells, and they include:
Cytotoxic T cells
Cytotoxic T cells, also called killer T cells and CD8+ T-cells, are T cells that scan the surfaces of cells to detect infections caused by bacteria, viruses, and even cancer. Cytotoxic T cells destroy the cells that are infected and help prevent the development of autoimmune diseases. However, abnormal activities of cytotoxic T cells can give way to the persistence of pathogens including autoimmune disorders.
Helper T cells
The body's immune response can be activated or controlled with the help of helper T cells. These cells help secrete cytokines, activate cytotoxic T cells, and support the maturation of B cells. However, helper T cells only work when they are activated on the surface of antigen-presenting cells (APCs).
Helper T cells are also categorized into groups according to their:
- Th1 response: It is characterized by interferon-gamma release and is more effective when it comes to fighting off bacteria and viruses that internally infect the cells.
- Th2 response: It is characterized by interleukin-5 release and is more effective in fighting off pathogens that externally infect cells such certain parasites and bacteria.
- Th17 cells: These cells are the inflammatory counterparts of regulatory T cells.
Regulatory T cells
Although regulatory T cells can suppress immunity for tumors and certain pathogens, they also limit inflammatory conditions, maintain tolerance, and prevent the development of autoimmune disorders.
Memory T cells
Memory T cells are the ones that help the immune system remember past infections. They usually survive for a longer time after the body recovers from an infection, and when they are re-exposed to past infections, they rapidly multiply to eliminate such infections. Memory T cells are also important when it comes to vaccine development.
Natural Killer T (NKT) cells
Natural killer T cells produce cytokines, regulate immune responses against autoantigens, and help connect innate and adaptive immune systems.
2. B cells
B cells mature in the bone marrow and secrete cytokines and antibodies. These cells usually function in the adaptive immune system. B cells work with molecules or antigens that can trigger an immune response to effectively produce antibodies.
Memory B cells
These cells circulate throughout the body to quickly initiate an antibody response when an antigen is detected. Memory B cells help the body's immune system to have a faster response when reinfection occurs.
Regulatory B cells
Regulatory B cells help promote regulatory T cell generation. These cells help stop the lymphocytes that cause inflammation. An abnormal recognition of B cells, as well as an abnormal B cell transformation, can cause autoimmune diseases, such as lupus, arthritis, multiple sclerosis, diabetes, and cancer.
3. Natural Killer (NK) Cells
NK cells play a role in the innate immune system. NK cells provide a quick response to cells that are infected. Moreover, these cells do not require antibodies to start immune reactions. They also help in fighting secondary infections. NK cells are particularly needed when it comes to fighting cancer since they might show innate resistance to certain viruses such as HIV.
White Blood Cell (WBC) Count
A white blood cell (WBC) count is a simple blood test that can measure the number of WBCs including lymphocytes in the body. Abnormal WBC counts are shown by either a higher or lower number than the normal range for a person's age.
Abnormal results may indicate certain medical conditions or blood disorders. The doctor may consider several factors to determine the exact cause of the abnormal result. These factors include a patient's medical history, symptoms, and specific medications taken.
Other Roles of Lymphocytes
- Protection against cancer - The overall survival of a cancer patient can be determined when the level of T cells are higher. The symptoms of liver cancer can be treated and malignant tumors can be prevented from recurring by using specialized tumor-infiltrating lymphocytes therapy.
- Intestinal health is maintained - The lymphocytes in the gut play a significant role when it comes to maintaining gut homeostasis. Lymphocytes are also needed to have an early response to infections in the gastrointestinal tract.
- Protection against arthritis - It was found in one study that cartilage and bone damage was lesser in patients who have high lymphocyte levels in their joints compared to those who have lower levels of lymphocytes in the joints.
- Lymphocytes and blood pressure - CD8(+), Th1, Th17, and T regulatory T cells may have numerous effects on blood pressure.