A lymphocyte is a type of white blood cell (leukocyte) that is a fundamental component of the immune system. Lymphocytes originate from hematopoietic stem cells in the bone marrow and are central to the body's ability to recognize and mount a tailored defense against specific pathogens like viruses, bacteria, and fungi, as well as to destroy cancer cells. Janeway's Immunobiology, 9th ed. They constitute approximately 20-40% of the circulating white blood cells in adult humans.

Types of Lymphocytes

Lymphocytes are broadly classified into three major types, each with distinct functions.

T Cells (Thymus-derived cells)

T cells are so named because they mature in the thymus gland. They are the primary agents of cell-mediated immunity. There are several subtypes of T cells:

• –Helper T cells (CD4+): These cells coordinate the immune response. Upon activation, they release cytokines that signal and direct other immune cells, including B cells and cytotoxic T cells.
• –Cytotoxic T cells (CD8+): Also known as killer T cells, these cells directly kill virally infected cells, tumor cells, and cells that are damaged in other ways by recognizing specific antigens presented on their surface.
• –Regulatory T cells (Tregs): These cells suppress the immune response, helping to prevent autoimmune reactions and maintain immune system homeostasis.
• –Memory T cells: Long-lived cells that 'remember' a specific antigen, allowing for a faster and stronger immune response upon subsequent exposure.

B Cells (Bone marrow-derived cells)

B cells are responsible for humoral immunity, which involves substances found in the body's humors (body fluids). Lymphocyte - Encyclopædia Britannica When a B cell encounters its specific antigen, it can be activated—often with help from a helper T cell—to differentiate into two main cell types:

• –Plasma cells: These are effector B cells that function as antibody factories, producing large quantities of antibody molecules that can neutralize pathogens, mark them for destruction, or activate other parts of the immune system.
• –Memory B cells: Similar to memory T cells, these cells persist after an infection is resolved, providing long-term immunity.

Natural Killer (NK) Cells

Natural killer cells are a component of the innate immune system, meaning they do not require prior sensitization to an antigen to act. They provide a rapid response by identifying and killing cells that lack the normal 'self' markers (MHC class I molecules) on their surface, a common characteristic of some tumor cells and virally infected cells. Lymphocyte - National Cancer Institute Dictionary

Development and Lifecycle

All lymphocytes arise from common lymphoid progenitors in the bone marrow. B cells complete their maturation within the bone marrow, while immature T cells, known as thymocytes, migrate to the thymus for their maturation process. Once mature, these 'naïve' lymphocytes circulate throughout the body in the blood and lymph. They patrol for pathogens, passing through secondary lymphoid organs such as the spleen and lymph nodes, where they are most likely to encounter foreign antigens presented by other immune cells like macrophages and dendritic cells.

Clinical Significance

Measuring the number of lymphocytes in the blood can be a valuable diagnostic tool.

• –Lymphocytosis: An abnormally high level of lymphocytes in the blood. It is often a normal, temporary response to an infection (e.g., mononucleosis). However, persistent lymphocytosis can indicate a chronic infection or certain types of cancer, such as chronic lymphocytic leukemia (CLL) or acute lymphoblastic leukemia (ALL).
• –Lymphocytopenia: An abnormally low level of lymphocytes. This condition weakens the immune system, increasing susceptibility to infections. Causes can be acquired, such as through HIV infection which specifically targets CD4+ helper T cells, malnutrition, or use of immunosuppressive drugs. It can also be hereditary. Lymphocytopenia - Merck Manual
• –Malignancies: Cancers that arise from lymphocytes are broadly known as lymphomas (if they form solid tumors in the lymphatic system) and lymphocytic leukemias (if they are primarily in the blood and bone marrow). Examples include Hodgkin's lymphoma and non-Hodgkin lymphoma.

Research and Therapeutics

The central role of lymphocytes in immunity has made them a prime target for modern medical therapies. In the field of oncology, CAR-T cell therapy is a revolutionary form of immunotherapy where a patient's own T cells are extracted, genetically engineered to produce chimeric antigen receptors (CARs) that recognize cancer cells, and then infused back into the patient to attack the tumor. CAR T Cells: Engineering Immune Cells for Cancer Therapy