The functional anatomy of organisms is maintained by the coordination of different systems, that often rely on particular interactions between specialized cells and between macromolecules. The immune system works with the circulatory and the lymphatic systems to protect most of the organs. However, some organs are considered immune privileged due to the presence of highly selective and regulated barriers, such as the blood-brain barrier (BBB) within the brain. The BBB controls periphery-brain molecule exchange and prevents immune effector cells from entering the homeostatic brain. BBB-associated elements, such as endothelial cells, pericytes, astrocytes, and microglia, potentially can function as antigen-presenting cells (APC). Pathological scenarios that induce dysfunction of the BBB and its associated cells may lead to the infiltration of lymphocytes crossing over from the blood to brain. Similarly, traumas can also enable B and T lymphocytes to pass bidirectionally between the CNS and the periphery, via the meningeal lymphatic vessels which drain into the cervical lymph nodes. Research in animals and in humans has revealed that B and T cells are involved in the progression of neurological diseases. It has been showed that under particular conditions, T cells establish themselves and become resident in the brain (T RM cells), from where they can either exert beneficial or detrimental effects on brain function. Amazing efforts have been made to further comprehend interactions between brain-specific cells and peripheral immune cells, and especially their roles and impact on the onset, progression, and eventual resolution of diverse brain pathologies.