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©2013 UpToDate ® Print Email Representation of T cell activation Schematic representation of initiation of the immunologic response to an antigen. The antigen binds to a groove in MHC class II molecules on antigen-presenting cells (APCs, such as macrophages). This binding allows the antigen to be presented to antigen receptors on autoreactive CD4 inducer or helper T cells which, in type 1 diabetes mellitus, initiate autoimmune injury to the pancreatic beta-cells. In addition, the respective binding of B7 proteins and LFA-3 (lymphocyte functional antigen-3) on APCs to CD28 and CD2 on T cells are important costimulatory pathways that further increase T-cell activation. Other molecules also can participate in the immune response, such as the binding of interleukin-2 to its receptor (IL-2R).