That system is called the immune system
One of the biggest concerns in doctors that I have seen from watching TV is that flu pandemics if left untreated can turn deadly not within a matter of days but within a matter of minutes. Hope this helped.
They can make friends there, and feel like they are worth talking to, which makes them feel happier.
Also, when getting a question right, or maybe getting a good grade on a worksheet, can make them feel smart, which also makes them feel happier.
Hope this helps, and happy studying~!
~{Dunsforhands}
A is the correct answer. A university study
