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.
Template drawing from "The Immune System", any modifications, made by myself are released into the public domain.
<a rel="nofollow" class="external text" href="http://www.niaid.nih.gov/publications/immune/the_immune_system.pdf">The Immune System (pdf)</a>