**5. Microglia**

Microglia are the resident macrophages in brain. These cells are believed to be derived from bone marrow, and present in brain from birth with no replenishment of these cells during the life of an individual (Williams and Hickey, 1995). In normal, healthy brain, microglia play a surveillance role. The high surface area to volume ratio is indicative of a cell "sampling" its environment (Figure 2). On activation, fine processes are no longer visible, with the microglia taking on a more amoeboid morphology. In SIV infection microglia can be recruited and productively-infected themselves (Gonzalez-Scarano and Martin-Garcia, 2005). These cells can also be induced to upregulate CD163 (Roberts et al., 2004b, Borda et al., 2008) which can be quite prominent in areas of BBB breakdown.
