illustration (IMAGE) UT Southwestern Medical Center Caption Activation of T cell immune response with the interaction of MHC-II (red) with the T cell receptor (TCR, blue), and also CD4 (light blue). CD4 and MHC-II are proteins expressed by T cells and antigen-presenting cells, respectively, to aid the recognition of antigens by TCRs. Credit UT Southwestern Medical Center Usage Restrictions None License Original content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.