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Lymphocytes, also known as lymphocytes, are a type of white blood cell that forms a core component of the immune system. They are classified into several subtypes, including T cells, B cells, and natural killer (NK) cells, each with specialized functions. T cells help coordinate immune responses, B cells produce antibodies to neutralize pathogens, and NK cells target infected or cancerous cells. Lymphocytes recognize and respond to foreign substances (antigens), such as bacteria, viruses, and toxins, initiating immune reactions to eliminate threats. They originate from hematopoietic stem cells in the bone marrow, where they differentiate into various lymphocyte types before migrating to lymphoid organs (like lymph nodes, spleen, and thymus) or circulating in the blood. Abnormalities in lymphocyte function or numbers can lead to immune disorders (e.g., immunodeficiency) or diseases (e.g., leukemia, lymphoma). Lymphocytes are essential for maintaining immune surveillance and protecting the body against infections and malignancies.