How are helper T-cells activated?

Helper T-cells feature a CD4 receptor on their cell membrane surface. This receptor interacts with Major Histocompatibility Complex (MHC) II molecules that can sense when there is a foreign substance, infection, or abnormal cells in the body. To activate helper T cells, CD4 receptors and MHC class II molecules release cytokines that send messages to other cells in the immune system. This initiates and coordinates an effective immune response.