It has long been a mystery as to why a small fraction of people exposed to HIV can stay healthy despite their contact with the virus that can eventually lead to AIDS.

“Most people exposed get infected, but a few do not,” said Reuben Harris, a biochemistry, molecular biology and biophysics professor at the University of Minnesota.

Harris and a team of University researchers recently delved into that question and discovered why HIV does not affect each person exposed to it. The results of their study were published late last month.

HIV comes in two forms —HIV-1 and HIV-2 — according to the National Center for Biotechnology Information. HIV-1 patients are more likely to progress in their disease and develop AIDS, whereas the virus’ second strain is characterized by lower transmissibility and likelihood of progression.

When a person is infected with HIV, Harris said, the virus attacks their immune system cells, called T cells, and replicates itself until it has killed off the bulk of those cells.

“HIV does [this] quite effectively,” he said.

But a small number of people naturally resist HIV infection because of a genetic variation in a cell surface protein, which can prevent the virus from binding to and infecting human cells.

That variant protein’s presence only explains less than 20 percent of natural immunity to HIV, Harris said.

So his team turned its focus to finding what other factor could explain some patients’ apparent immunity HIV.

They found that the stable existence of a separate specific protein can, in certain cases, block HIV from replicating.

That specific protein’s presence can range widely, Harris said.

He said that many non-Caucasians, especially those of African descent, do have stable forms of the protein that are better at fending off HIV infection. However, it’s unstable in many Minnesotans because of their European ancestry, he said.

He said researchers plan to test couples in which one partner is HIV-1 positive and the other is not.

“All research is built on backs of previous research,” he said.

Harris also said it’s pressing for researchers to explore methods like drug therapies for strengthening human proteins that play a role in the likelihood of contracting HIV.

Eric Refsland, who worked on Harris’ HIV research team over the last two years of his doctoral studies, said there is still plenty unknown about the virus that affects more than 1.2 million Americans.

“The next steps are about how to manipulate a treatment,” he said. “The lab is going in the direction on how to find the global percentage of exposed HIV patients that are infected and not infected.”