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.
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.
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.
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