Keywords: T cell activation.png T cell activation svg The T lymphocyte activation pathway is triggered when a T cell encounters its cognate antigen coupled to a MHC molecule on the surface of an infected cell or a phagocyte T cells contribute to immune defenses in two major ways some direct and regulate immune responses; others directly attack infected or cancerous cells http //www niaid nih gov/publications/immune/the_immune_system pdf The Immune System pdf NIH Publication No 03 “5423 September 2003 modifications September 4 2006 Template drawing and caption text from The Immune System any modifcations made by myself are released into the public domain Original work of the US Federal Government - public domain Image T_cell_activation-tr png Image T_cell_activation-es png PD-USGov DO11 10 Microbiology Biology diagrams Immunology T cells |