Tcells often function to decrease the immune response release b7 proteins function in the adaptive immune system activation bind tightly to target cells and release a lymphotoxin called perforin

Helper T cells are very important cells in the humans immune system. They help activate B cells to secrete antibodies and macrophages to destroy ingested microbes and  cytotoxic T cells to kill infected target cells. So, according to this Helper T cells function in the adaptive immune system activation. 

Cells of an organism tell each other when predators are near.