Molecular markers of astrocytes. Although antigenically distinct from other cell types by virtue of its expressing GFAP [17], there is no documented evidence of astrocytic disease related to an immunological response to GFAP on any astroglial molecule. GFAP remains singularly the most used cytoplasmic marker of astrocytes. A reliable marker for astrocytic membranes remains to be described. Interestingly, there is increasing evidence demonstrating the ability of astrocytes to serve as accessory cells of the immune system in a number of immune-mediated conditions [27,28]. In this regard, astrocytes are known for their ability to express class II MHC antigens in vitro, which are molecules essential for the presentation of antigen to helper/inducer CD4+ T cells, as well as their ability to synthesize a number of cytokines, such as interleukin-1, tumor necrosis factor and interferon γ (see Chaps. 35 and 39). It appears, therefore, that in circumstances in which the BBB is interrupted, the astrocyte is a facultative phagocyte with the potential to interact with lymphocytes.