Scientists say the development of an AIDS vaccine might be more complicated than they thought, after researchers in California reported that the immune response to HIV differs, even in identical twins.

Scientists followed the cases of twin boys who were infected shortly after birth in 1983 through blood transfusions from the same HIV-positive donor. They were exposed to the same environmental factors as they grew up in the Los Angeles area. Their immune systems targeted the virus in similar ways.

But their T-Cell Receptors - which act as part of the body's attack forces - responded differently. Otto Yang, professor at University of California Los Angeles Medical School, was the lead researcher on the project. He says, "What was different was the structure of the T-Cell Receptors," and adds, "Even thought the T-Cell receptors were recognizing the same portions of the virus, they actually were doing it in slightly different ways."

The finding - reported in the Journal of Virology - shows that the interaction between the body's immune system and the virus is random and unpredictable, which means that a 'one size fits all' vaccine may not be possible.