AIDS: Scientists have identified a number of chemical messengers that play a fundamental role in directing the body's natural response to the invasion of organisms and pathogens in the human immune system. One group, cytokines, are an essential communications link between the cells of the immune system and the body. The decrease in immune function with HIV is apparent in both the amount of HIV detected and in the decline in CD4 cells. Levels of two cytokines, IL-12 and IL-2, appear to have a direct correlation to cellular immunity. Treatment with these two substances may strengthen immune function and decrease the pace of disease development. Recombinant forms have been developed as hopeful treatments for cancer, HIV, and other infectious diseases.