Lymphocyte

A lymphocyte is a type of white blood cell, an important part of the immune system that helps the body defend against invading organisms. Lymphocytes (including their two main types, B lymphocytes and T lypmphocytes) originate in the soft inside of the bones (the bone marrow) and migrate through the body to various sites, where they mature, differentiate, and monitor against any potential attack. B lymphocytes, or B cells, which mature in the bone marrow, produce a protein called an antibody that directs other cells to attack foreign invaders, such as bacteria or viruses. T lymphocytes, or T cells, which mature in the thymus, attack the body’s own cells if they have been infected by an invader or have become cancerous. Some lymphocytes have the ability to “remember” the target of an attack and produce antibodies that can make the body immune to future illnesses from those same invaders. The lymphocytes also have another subset of specialized cells called the natural killer (NK) cells, which are innately primed to identify and destroy cancerous and virus-infected cells.

Background

Blood is a fluid that courses through the body via a system of blood vessels called arteries and veins. It has a number of vital functions, including carrying oxygen to the cells of the body and defending against infection. Blood is made up of four components. One of those, plasma, is mostly water-based and acts as a medium to transport blood cells throughout the body. Red blood cells are biconcave disk-shaped cells that contain a protein called hemoglobin. Hemoglobin absorbs oxygen from the lungs and carries it to the other cells in the body. It also removes carbon dioxide waste and transports it back to the lungs where it can be exhaled. Platelets are cell-like structures that clump together to form tiny clots in the blood to stop bleeding.

White blood cells are the body’s natural infection fighters. These cells, also called leukocytes, defend the body against foreign invaders such as bacteria, viruses, fungi, or parasites. White blood cells, along with platelets, make up a small fraction of blood, together forming less than 1 percent of total blood volume, and they are divided into five types. Monocytes are the largest white blood cells and act as cell-devouring scavengers. They engulf and digest foreign organisms and even other dead white blood cells. Eosinophils release toxins that primarily target parasites, such as intestinal worms. High eosinophil counts are linked to allergic and asthmatic reactions. Basophils are the least common type of white blood cell and act as a warning system to alert the body to the presence of invading organisms. They do this by releasing an organic compound called histamine, which widens the blood vessels and allows other white blood cells to reach the site of the infection. Like eosinophils, they are also associated with allergies. Neutrophils are the most common type of infection fighter, comprising about 50 to 70 percent of the body’s white blood cells. They function as natural assassins, killing their targets (pathogens) through various methods.

Overview

Lymphocytes, which make up about 20 to 40 percent of the body’s white blood cells, are the fifth type. They originate in the soft tissue inside the bones called the marrow. From there, they migrate out through the body by way of the lymphatic system, a network of tissues and organs that includes the lymph nodes, spleen, and thymus. The lymphatic system helps filter out toxins, bacteria, and other foreign substances from the body. Depending on their destination, the lymphocytes will become specialized into one of two types—B lymphocytes or T lymphocytes—both of which have the same goal of defending the body against foreign invaders. When the body senses an invading organism, it recognizes it by its antigen, the shape of the molecules on its surface. The immune system then produces specific lymphocytes with specialized receptors on their surfaces designed to battle a specific antigen.

About three-quarters of the lymphocytes travel to the thymus where they become T cells. The T cells get their name from the thymus, a gland located in the center of the chest between the lungs. The T cells themselves are also specialized into two types. Helper T cells recognize an invading antigen and send out chemical signals that trigger B cells and killer T cells into action. When a killer T cell encounters the specific antigen it was designed to fight, the helper T cell activates the killer cell to make copies of itself. These killer T cells use the receptors on their surface to identify body cells that have been infected by an invader or have become cancerous. If an antigen is detected, the killer T cells destroy the cell. Some T cells remain in the body long after the immune system has finished responding to the infection in case they are needed to deal with a recurrence. Advances in immunotherapy have led to the development of chimeric antigen receptor (CAR) T-cell therapy, in which a patient’s T cells are genetically engineered to better recognize and destroy cancer cells.

B cells make up about a quarter of the body’s lymphocytes. They do not attack invading organisms. Instead, they attach themselves to an invader when a matching antigen is detected, and often await instructions from helper T cells, although some can be activated independently. When they receive the signal, the B cells begin dividing, creating plasma cells and B memory cells. Plasma cells produce special proteins called antibodies. These antibodies are tailor-made to target the same antigen as their parent cell. They coat an infected cell or invading organism and mark it for destruction, letting killer T cells or infection-devouring cells called macrophages know which cells to target. Antibodies can also attach themselves to several specific antigens at once, clumping together groups of invaders and making them easier to devour.

B memory cells have a long lifespan and are able to retain a memory of the receptor pattern of the antigen they were programmed to fight. Studies have shown that memory B and T cells can persist after an infection and continue to contribute to long-term immune protection even when antibody levels decline. Hence, if the same foreign organism enters the body, the B memory cells remain active to fight the invader. This function of lymphocytes is responsible for the body developing immunity against certain illnesses.

An abnormally high number of lymphocytes in the blood can lead to a condition known as lymphocytosis. In clinical practice, lymphocytosis in adults is typically defined as a lymphocyte count exceeding about 4,000 cells per microliter of blood. Because lymphocytes are part of the body’s natural defense system, it is not unusual for their count to be elevated while the body deals with an infection. If the lymphocyte levels remain high, however, it may signal the presence of a more serious illness, such as leukemia, HIV/AIDS, cancer of the lymphatic system, or a severe bacterial infection.

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