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 bloodbrain barrier, bloodcerebrospinal fluid barrier, and similar fluidbrain 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.

Answer this series of questions:

1. what is the article about?
2. how is it defined?
3. what must it do to work correctly?
4. are they also called something else?
5. is the worm helpful?
6. do pathogens change?
7. slowly?
8. how are brains protected?
9. are they solid?
10. what are they composed of?
1. The immune system
2. system of many biological structures
3. detect a wide variety of agents
4. pathogens
5. No
6. Yes
7. No
8. brain barriers separate the peripheral immune system from the neuroimmune system
9. No
10. blood and fluids