A<unk>  is a type of lymphocyte.<unk> s are one of the important white blood<unk> s of the immune system and play a central role in the adaptive immune response.<unk> s can be distinguished from other lymphocytes by the presence of a<unk>  receptor (TCR) on their<unk>  surface.<unk> s are born from hematopoietic stem<unk> s, found in the bone marrow. Developing<unk> s then migrate to the thymus gland to develop (or mature).<unk> s derive their name from the thymus. After migration to the thymus, the precursor<unk> s mature into several distinct types of<unk> s.<unk>  differentiation also continues after they have left the thymus. Groups of specific, differentiated<unk>  subtypes have a variety of important functions in controlling and shaping the immune response. One of these functions is immune-mediated<unk>  death, and it is carried out by two major subtypes: CD8+ "killer" and CD4+ "helper"<unk> s. (These are named for the presence of the<unk>  surface proteins CD8 or CD4.) CD8+<unk> s, also known as "killer<unk> s", are cytotoxic – this means that they are able to directly kill virus-infected<unk> s, as well as cancer<unk> s. CD8+<unk> s are also able to use small signaling proteins, known as cytokines, to recruit other types of<unk> s when mounting an immune response. A different population of<unk> s, the CD4+<unk> s, function as "helper<unk> s". Unlike CD8+ killer<unk> s, the CD4+ helper T (TH)<unk> s function by further activating memory<unk> s and cytotoxic<unk> s, which leads to a larger immune response. The specific adaptive immune response regulated by the<unk>  depends on its subtype, which is distinguished by the types of cytokines they secrete. Regulatory<unk> s are yet another distinct population of<unk> s that provide the critical mechanism of tolerance, whereby immune<unk> s are able to distinguish invading<unk> s from "self". This prevents immune<unk> s from inappropriately reacting against one's own<unk> s, known as an "autoimmune" response. For this reason, these regulatory<unk> s have also been called "suppressor"<unk> s. These same regulatory<unk> s can also be co-opted by cancer<unk> s to prevent the recognition of, and an immune response against, tumor<unk> s.
T cell
Dendritic cell
Natural killer cell
Mast cell
Reactive lymphocyte
Prolymphocyte
Cytokine-induced killer cell
Monocyte