The activation of T cells and B cells by an antigen. The antigen is process by the antigen-presenting cell (APC) and presented to the T cell. Some activated T cells stimulate B cells to produce humoral immunity (antibodies) and some produce cellular immunity (mediated by T cells and macrophages). Both T cells and B cells also produce memory cells to mediate a rapid response to later exposure to the antigen. Credit: Mikael Häggström, used with permission. [Public domain], via Wikimedia Commons