A lymphocyte is a type of white blood cell involved in the vertebrate immune system. There are two broad categories of lymphocytes, namely the large granular lymphocytes and the small lymphocytes. The large granular lymphocytes are more commonly known as the natural killer (NK) cells. The small lymphocytes are the T cells and B cells. Lymphocytes play an important and integral role in the body's defenses.

Natural Killer (NK) cells are a part of cell-mediated immunity and act during the innate immune response. They can attack host cells that display a foreign (e.g. viral) peptide on particular cell surface proteins known as MHC class I molecules. Once they determine a cell is infected, the NK cells release cell killing (cytotoxic) granules that will destroy the infected cell. NK cells do not require prior activation in order to perform their cytotoxic effect upon target cells. Like NK cells, the T cells are chiefly responsible for cell-mediated immunity whereas B cells are primarily responsible for humoral immunity. T cells are named such because these lymphocytes mature in the thymus; B cells (named for the bursa of Fabricius in which they mature in bird species) are thought to mature in the bone marrow in humans. T and B lymphocytes differ from NK cells in that they are the principal cells involved in the adaptive immune system. These are cell types that retain a memory of a previous infection so that they can respond to the same infectious agent quickly upon reinfection. In the presence of an antigen, B cells can become much more metabolically active and differentiate into plasma cells, which secrete large quantities of antibodies. T cells, after they see an antigen, will also become highly activated and will secrete specific proteins, such as cytokines and cytotoxic granules, depending on their subtype or function.