Two proteins found on HIV's outer surface or envelope, known as gp160 and gp120, have been produced in different expression systems, including yeast, insect, and mammalian cells. These recombinant proteins have been tested in combination with several substances, called adjuvants, that are designed to improve immune responses. Such DNA vaccines against influenza and HIV are now in early clinical tests. It is unknown what immune responses will prevent or lessen the severity HIV infection, but other vaccines can protect against infection or against disease by inducing neutralizing antibodies that prevent the virus from entering cells. This form of protection, known as humoral immunity, would be effective against the virus when it is not inside a cell.
Most vaccines probably work through a combination of antibodies and cellular immunity.
However, the importance of cellular immunity in protection from chickenpox is shown by the much greater susceptibility of people whose cellular immune system is weakened, for example, by medicines that treat cancer or prevent rejection of transplanted kidneys. Because sexual and some perinatal transmission of HIV occurs at mucosal surfaces that have their own forms of immune protection, it may also be important to stimulate specific antibodies in those areas. Immune responses at these sites are now being studied in animals and humans. Vaccines have been designed to be administered via the nose, mouth, vagina, or rectum to concentrate the immune response at these vulnerable areas. Vaccine development also will have to take into account HIV-1's extraordinary variability.