Activation of T Cells: Helper
Helper T cells only recognize antigen in the grasp of Class II MHC markers. An antigen-presenting cell--such as a macrophage or a dendritic cell--breaks down the antigen it devours, then it places small pieces (peptides) on its surface along with a Class II MHC marker. By exhibiting its catch in this way, antigen-presenting cells enable specific receptors on helper T cells to bind the antigen and confirm (via CD4 protein) that an invasion has occurred.
After binding, a resting helper T cell quickly becomes an activated helper T. It assumes command of the immune response, giving orders to increase the number of specific antibody-producing plasma cells and the cytotoxic killer cells needed to quell the attack.