About 30 percent of adults with AIDS eventually develop neurological disorders, including dementia. If you look at the brain of someone who had such dementia, you see clumps of HIV-infected macrophages. (Macrophages are a kind of immune cell that floats around the body gobbling up foreign material.) AIDS researchers have been debating whether the virus gets into the brain early on but doesn't do anything for a while, or whether the macrophages carry it to the brain later in the infection.

What they wanted to know: What causes dementia in people infected with HIV?

What they did: The scientists analyzed slices of brain tissue from nine patients with HIV and dementia, three with HIV but no dementia, and five without HIV. The brains were from people who died at 22 to 50; most were men in their 30s and 40s. The brain slices were doused with antibodies that glow under certain lights if they're attached to macrophages, so researchers could tell how many of several kinds of macrophages were in the brains.

What they found: The researchers think the macrophages come from outside the brain. As in other studies, patients with HIV encephalitis, which includes dementia, had a lot of macrophages in their brains. But the scientists didn't find a chemical that macrophages display when they are multiplying, suggesting that the macrophages came in from outside the brain, instead of multiplying in place.

What it means to you: If the macrophages are coming from outside, that helps explain why antiretroviral drugs help patients with HIV dementia—even though the drugs don't get into the brain very well.

Caveats: The researchers looked at a total of only 17 brains—but they did find big enough differences between them to get statistical significance.

Find out more: From the National Library of Medicine: http://www.sis.nlm.nih.gov/

Funded by the National Institute of Neurodegenerative Disorders and Stroke, http://www.ninds.nih.gov/, and the Howard Hughes Medical Institute, http://www.hhmi.org

Researcher Jay Rappaport's web page: http://www.temple.edu/biology/faculty/jrappaport.html

Read the article: Fischer-Smith, T. et al. "Macrophage/Microglial Accumulation and Proliferating Cell Nuclear Antigen Expression in the Central Nervous System in Human Immunodeficiency Virus Encephalopathy." American Journal of Pathology. June 2004, Vol. 164, No. 6, pp. 2089-2099.