B. Dietzschold & J.A. Richt 
Protective and Pathological Immune Responses in the CNS [PDF ebook] 

In spite of the protection of the CNS there are situations where immune responses occur due to different disease processes. While antigen-specific cells of the adaptive immune response are not normally functional in the CNS tissue, CNS resident cells respond to infection or insult. One mechanisms is through the induction of apoptosis in virus infected neurons, which may be protective or pathogenic depending on the extent of cell death.