<P> The immune system is a host defense system comprising 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 . </P> <P> Pathogens can rapidly evolve and adapt, and thereby avoid detection and neutralization by the immune system; however, multiple defense mechanisms have also evolved to recognize and neutralize pathogens . Even simple unicellular organisms such as bacteria possess a rudimentary immune system in the form of enzymes that protect against bacteriophage infections . Other basic immune mechanisms evolved in ancient eukaryotes and remain in their modern descendants, such as plants and invertebrates . These mechanisms include phagocytosis, antimicrobial peptides called defensins, and the complement system . Jawed vertebrates, including humans, have even more sophisticated defense mechanisms, including the ability to adapt over time to recognize specific pathogens more efficiently . Adaptive (or acquired) immunity creates immunological memory after an initial response to a specific pathogen, leading to an enhanced response to subsequent encounters with that same pathogen . This process of acquired immunity is the basis of vaccination . </P>

How many immune systems does the body have
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