The system could be a host weaponry comprising several biological structures associate degreed processes at intervals an organism that protects against malady. To operate properly, associate degree system should sight a good sort of agents, referred to as pathogens from viruses to parasitic worms and distinguish them from the organism's own healthy tissue. In several species, there are a unit 2 major subsystems of the system: the innate system and therefore the adaptational immune system each sub systems use body substance immunity and cell-mediated immunity to perform their functions. In humans, the blood–brain barrier, blood– cerebrospinal fluid barrier, and similar fluid–brain barriers separate the peripheral system from the neuroimmune system, that protects the brain.
