T helper cells are an important part of the adaptive immune response. By recognising foreign antigen presented on the surface of self-cells by MHC II. By internalising this antigen and presenting it on its surface, the T helper cell is able to activate a complementary B cell when it moves through the lymphoid tissue (via contact-dependent interactions with the B cell receptor).
T helper cells produce interleukins, which are able to activate other parts of the immune system. Different types of T helper cell produce different interleukins. For example, T helper 17 produces IL-17. This recruits neutrophils, inducing inflammation at the site of infection.