Neuroimmunology is a field combining neuroscience, the study of the nervous system, and immunology, the study of the immune system. Neuroimmunologists seek to better understand the interactions of these two complex systems during development, homeostasis, and response to injuries. Unlike other organs of the body, the brain is more susceptible to immune system-mediated damages, as chemicals released by immune cells as a part of the defense mechanism are often very toxic to brain cells. These chemicals can damage the cells of the central nervous system and disrupt neuronal circuits. In addition, inflammation caused by the immune system can also induce damages to brain cells, which are secondary to the original injury or infection.The blood-brain barrier which restricts, or selectively allows, the entry of substances from bloodstream, plays a crucial role in protecting the brain from unwanted damages caused by the immune system. One classical example in this regard is how B lymphocyte (B cell) exerts its action on the brain. B cells are one type of immune cell that produce and secrete antibodies against foreign invaders (antigens) to selectively destroy them. These cells cannot enter the brain freely because the blood-brain barrier prevents the free-entry of large molecules, including B cells, from bloodstream into the brain. B cells can migrate to the brain only if an immune response is activated in some other parts of the body. In this way, the brain is protected from unnecessary damages.