Persistent pathogens in the parenchyma of the brain.

It has recently been shown that bacteria and viruses can be delivered to the brain parenchyma without evoking an immune response. These experiments demonstrate that there are no cells within the brain parenchyma that can initiate a primary immune response, and that the drainage of pathogens from the brain parenchyma is distinct from that documented for soluble proteins. A persistent pathogen in the brain parenchyma can become a target for the immune system following peripheral sensitisation, and this may lead to bystander tissue damage. These observations may have consequences for vaccination of persons with central nervous system HIV infection.