The adaptive immune system is in part regulated by a vast array of co-stimulatory molecules which prevent undue activation of lymphocytes such as B and T cells. Similarly, co-inhibitory molecules serve a similar purpose by providing a check against dysregulated activation. Together, these to types of molecules function to help control the adaptive immune response generated by foreign or self antigens.

Listed below are common families and classes of these molecules, along with markers which may be used to identify them.