The Brain's Immune System; November 1995; Scientific American Magazine; by Streit, Kincaid-Colton; 6 Page(s)

When biologists view healthy tissue from the brain or spinal cord under a microscope, they rarely see white blood cells, the best known sentries of the immune system. And for good reason. Although white blood cells defend against infection and cancer, they also can secrete substances capable of killing irreplaceable nerve cells, or neurons. The body minimizes such destruction by restricting the passage of immune cells out of blood vessels and into the central nervous system; white cells generally escape into the nerve tissue only when blood vessels are damaged by trauma or disease.

Such observations led to the once widespread belief that the central nervous system lacks immune protection. Recently, however, investigators have demonstrated that fascinating cells called microglia form an extensive defensive network there. Most of the time, microglia serve without harming neurons. Yet mounting evidence suggests they occasionally lose their benign character. In fact, there are intimations that the cells can help cause or exacerbate several disabling conditions, among them, stroke, Alzheimer's disease, multiple sclerosis and other neurodegenerative disorders.