Lymphocytes - what are, definition

What are Lymphocytes

A type of LEUKOCYTE (white BLOOD cell) that primarily resides in the LYMPH and lymph tissues. Lymphocytes are the body’s primary immune defense and move through the lymph in response to antigens and pathogens. When more rapid deployment is necessary, lymphocytes enter the bloodstream. About 1 percent of the body’s lymphocyte population circulates in the blood, making up about 25 percent of the circulating leukocytes. There are two major types of lymphocytes-T-cells and B-cells-and natural killer cells. Each type has different immune responsibilities.


T-cells, which make up about 75 percent of lymphocytes, originate in the BONE MARROW and migrate to the THYMUS to come to maturity. In the thymus T-cells acquire the ability to distinguish between “self” and “nonself,” an essential function of determining whether the particles the T-cells encounter are invaders. Mature T-cells carry kinds of antibodies, identified as CLUSTERS OF DIFFERENTIATION, that denote the T-cell’s immune function. There are numerous subtypes of T-cells, the most common being

  • helper T-cells, which secrete a cytokine called CD4 (for cluster of differentiation 4) that directs the response of other T-cells
  • cytotoxic T-cells, which attack invading cells by releasing chemicals that penetrate their cell membranes, which causes them to rupture and die
  • suppressor T-cells, which reign in the IMMUNE RESPONSE after the immune attack has squelched the threat
  • memory T-cells, which retain the ability to produce antibodies against the same ANTIGEN should it reappear in the body


B-cells, which make up about 10 percent of lymphocytes, originate in the bone marrow and migrate to the lymph tissues to come to maturity and await activation via contact with an antigen. When such contact occurs, the individual B-cell develops antibodies specific to the antigen, differentiates into either a memory B-cell or a PLASMA cell and then proliferates within the lymph tissues, lymph, and bloodstream. Memory cells “remember” the specific antigen and produce antibodies whenever the antigen again enters the body. This process provides long-term protection against pathogens. Plasma cells generate copious antibodies as they replicate, providing an immediate immune response to the PATHOGEN.

Natural Killer Cells

Natural killer (NK) cells are specialized lymphocytes that attack and destroy self cells that have become defective in some way. Researchers believe one function of NK cells is to attack tumors as they are beginning to develop, preventing them from taking root. NK cells also appear to attack cells that viruses hijack, preventing the VIRUS from replicating and causing infection.

For further discussion of lymphocytes within the context of blood and lymph structure and function please see the overview section “The Blood and Lymph.”


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The Blood and Lymph System

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