Antigen presentation stimulates T cells to become either "cytotoxic" CD8+ cells or "helper" CD4+ cells. Cytotoxic cells directly attack other cells carrying certain foreign or abnormal molecules on their surfaces. Helper T cells, or Th cells, coordinate immune responses by communicating with other cells. In most cases, T cells only recognize an antigen if it is carried on the surface of a cell by one of the body’s own MHC, or major histocompatibility complex, molecules.
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NIH Publication No. 03–5423, September 2003 (modifications: September 4, 2006)
{{Information| |Description= |Source=http://www.niaid.nih.gov/publication/immune/the _immune_system.pdf |Date=September 4, 2006 |Author=Template drawing from "The Imune System", any modifcations, made by myself are released into the public domain |Permiss