Helper T cells are activated when they recognize antigens presented by antigen-presenting cells (APCs) via Major Histocompatibility Complex (MHC) class II molecules. Once activated, they secrete cytokines that stimulate the activity of various immune cells. There are different subtypes of helper T cells, such as Th1, Th2, Th17, and T regulatory cells (Tregs), each producing distinct cytokines and playing unique roles in immune regulation.
