A lymphocyte is a type of white blood cell that plays an important role in the immune system. It is part of the body's defense against infections and diseases, as it helps to identify and destroy foreign substances such as bacteria, viruses, and other pathogens. Lymphocytes are produced in the bone marrow and mature into two main types: B cells (which produce antibodies) and T cells (which help regulate immune responses). There are also other types of lymphocytes such as natural killer cells, which can recognize and destroy infected or cancerous cells without the need for prior exposure. Overall, lymphocytes play a crucial role in maintaining health by protecting the body against disease-causing agents.