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. 

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. QUESTIONS:

Q1:
1. What is exposed in the article?
2. What is tackled by the article?
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Q2:
1. How is the immune system described?
2. How is the immune system referred to?
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Q3:
1. What is needed for the immune system to work correctly?
2. What must be done for the immune system to work correctly?
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Q4:
1. What is the other name of the agent?
2. What are the agents called?
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Q5:
1. Is the worm useful?
2. Does the worm help?
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Q6:
1. Are pathogen agents able to change?
2. Can pathogen agents change?
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Q7:
1. Do pathogen agents change slowly?
2. Do pathogen agents take time to change?
3. 
Q8:
1. What protects the brains?
2. What are the brains protected by?
3. 
Q9:
1. Are the brain barriers solid?
2. Are the brain barriers strong?
3. 
Q10:
1. What are the brain barriers made of ?
2. What composes the brain barriers?
3. 
