Editing Immune system (section)

===Recognition of antigen===
The cells of the adaptive immune system are special types of leukocytes, called lymphocytes. [[B cell]]s and T cells are the major types of lymphocytes and are derived from [[hematopoietic stem cell]]s in the [[bone marrow]].{{sfn| Janeway |2005 |p=}} B cells are involved in the [[humoral immunity|humoral immune response]], whereas T cells are involved in [[cell-mediated immunity|cell-mediated immune response]]. Killer T cells only recognize antigens coupled to [[Major histocompatibility complex#MHC class I|Class I MHC]] molecules, while helper T cells and regulatory T cells only recognize antigens coupled to [[Major histocompatibility complex#MHC class II|Class II MHC]] molecules. These two mechanisms of antigen presentation reflect the different roles of the two types of T cell. A third, minor subtype are the [[gamma/delta T cells|γδ T cells]] that recognize intact antigens that are not bound to MHC receptors.<ref name="Holtmeier W, Kabelitz D 2005 151–83">{{cite journal | vauthors = Holtmeier W, Kabelitz D | title = gammadelta T cells link innate and adaptive immune responses | volume = 86 | pages = 151–83 | year = 2005 | pmid = 15976493 | doi = 10.1159/000086659 | isbn = 3-8055-7862-8 | journal = Chemical Immunology and Allergy }}</ref> The double-positive T cells are exposed to a wide variety of [[Self-protein|self-antigens]] in the [[thymus]], in which [[iodine]] is necessary for its thymus development and activity.<ref name="pmid19647627">{{cite journal | vauthors = Venturi S, Venturi M | title = Iodine, thymus, and immunity | journal = Nutrition | volume = 25 | issue = 9 | pages = 977–79 | date = September 2009 | pmid = 19647627 | doi = 10.1016/j.nut.2009.06.002 }}</ref> In contrast, the B cell antigen-specific receptor is an antibody molecule on the B cell surface and recognizes native (unprocessed) antigen without any need for [[antigen processing]]. Such antigens may be large molecules found on the surfaces of pathogens, but can also be small [[hapten]]s (such as penicillin) attached to carrier molecule.{{sfn|Janeway|Travers|Walport|2001|loc= sec. [https://www.ncbi.nlm.nih.gov/books/NBK27112/#A1746 12-10]}} Each lineage of B cell expresses a different antibody, so the complete set of B cell antigen receptors represent all the antibodies that the body can manufacture.{{sfn| Janeway |2005 |p=}} When B or T cells encounter their related antigens they multiply and many "clones" of the cells are produced that target the same antigen. This is called [[clonal selection]].{{sfn | Sompayrac | 2019 | pp= 5–6}}