The AIDS virus evades the immune system because most of the proteins that cover the surface of the virus constantly change their structure. But researchers have now identified a site on the virus which does not change, and shown that antibodies which bind to it can protect monkeys from infection. The major antigen on the surface of HIV is a protein called gp120. During infection, gp120 binds to CD4, allowing the virus to enter cells. New research has found a key part of the gp120 which does not change and can be blocked by antibodies. Some people infected with HIV have similar antibodies, but because they have already been exposed to the virus, it is too late to prevent infection. Vaccines based on this new discovery will only work if they are given before people are exposed to HIV.