The number of brain mast cells increases under specific social, stressful, and disease states. Also important, is the discovery that brain mast cells can cross the BBB. Although mast cells are best known for their role in mediating allergic reactions, it has become increasingly evident that they also play a protective role in defense against bacterial infection. Mast cells are heterogeneous, and their mediator content is dependent on their microenvironment, suggesting that brain mast cells should be studied as a unique population - as distinct from those in the periphery. The present application proposes to develop a mouse model which will be used in understanding the phenomenology and functional consequences of mast cells in the brain. Pilot data indicate that mast cell deficient animals lack a complete Acute Phase Response (APR) to bacterial infection. To examine the involvement of mast cells in mounting an immune response, we explore 2 social/behavioral/endocrine conditions in which the brain mast cell population is augmented: a cohabitation paradigm and a stress paradigm. Next, we test the hypothesis that prior exposure (which increases brain mast cell numbers) results in altered response to challenge of the immune system in mast cell rich brain loci. Specifically, we ask whether the mast cell number, activation state is augmented when their numbers in the brain are elevated. To test the hypothesis that brain mast cells have immunological consequences, we determine if increased mast cell numbers results in altered T cell surveillance in mast cell rich brain regions. We propose to test these hypotheses in mast cell deficient mice, in their wild type littermates and in mast cell reconstituted animals. This application will determine whether mast cells play a role in mounting an immune response in the brain.