# Lymphocytes --- **Lymphocytes** are [[white blood cells]] that live primarily in the [[lymphatic system]]. These cells go around and patrol the whole body and look for foriegn invaders. They are [[B-cells]] and [[T-cells]] associated with the [[adaptive immune system]] and [[natural killer cells]] associated with the [[innate immune system]]. ## B-Cells & T-Cells [[B-cells]] and [[T-cells]] are two types of lymphocytes that share a lot of similarities, yet are definitely distinct from one another. ![[b-cells and t-cells.png]] Both B- and T-cells have receptors that bind to foreign molecules, though they are distinct in their shape and *how* they bind to the molecules. Each type of B or T-cell recognizes only a *single [[antigen|epitope]]*, so we have many many "varieties" of B and T-cells. Both cells also have [[cytokines]] receptors and [[MHC molecule]] molecules that they can use show off molecule to T-cells. In addition to their common features, B- and T-cells have some common characteristics as well. They both take time to activate, and they both can produce [[memory cells]]. Finally they both work closely with [[macrophages]], [[dendritic cells]], and [[natural killer cells]]. ## Natural Killer Cells The third main type of lymphocyte are the [[natural killer cells]]. They kill. Naturally. ## Lymphocyte Recirculation >One of the hurdles of developing adaptive immunity is the requirement of direct physical contact between antigen and the specific lymphocyte with the unique receptor that recognizes the antigen. It is estimated that only 1 in every 100,000 to 1 million T-lymphocytes or B- lymphocytes can bind with a specific antigen on the first exposure to that antigen—that is, during the antigen challenge. The “odds” for contact are increased because lymphocytes reside only temporarily in any given secondary lymphatic structure, and after a period of time they exit and then circulate through blood and lymph every several days. This process is referred to as lymphocyte recirculation. Lymphocyte recirculation provides a means of delivering different lymphocytes to secondary lymphatic structures, mak- ing it more likely that a lymphocyte will encounter its antigen, if present. ___