The
immune system is a system of many biological structures and
processes within an
organism that protects against
disease. To function properly, an immune system must detect a wide variety of agents, known as
pathogens, from
viruses to
parasitic worms, and
distinguish them from the organism's own healthy
tissue. In many species, the immune system can be classified into subsystems, such as the
innate immune system versus the
adaptive immune system, or
humoral immunity versus
cell-mediated immunity. In humans, the
blood–brain barrier,
blood–cerebrospinal fluid barrier, and similar fluid–brain barriers separate the peripheral immune system from the
neuroimmune system which protects the
brain.