The immune system is probably the most important and complex system in your body and should be studied more often in classrooms
People began making notes about the immune system as far back as 2000bc in Egypt and China when they noticed some people would have an adaptation to not get sick from disease they have had already or was native to them.
Your skin,mucous,and stomach acid are all part of the first line of defense to keep pathogens out of your body.
If somehow a pathogen was to enter the body then a second line of defense is triggered and it is your inflammatory response which basically brings fluids to the area to help "fight" the pathogen, and in these fluids are things called phagocytes.
Every cell in your body that has a nucleus has some means to protect and play a role in the immune system
When Columbus came to America he brought men,food,supplies,but he also brought something that would change the settlers lives and the lives of millions of native Americans.
He brought foreign sicknesses that would kill millions of Americans and what was sort of beneficial to the voyagers it would prove to be extremely detrimental to the natives.
If you've ever gone to get your flu shot it seems like no big deal but today their are vaccines for lots of sicknesses but a few hundred years of go and you get sick based on crazy theories so we have clearly came a long way and we now know how the system works and can get a better understanding of how to prevent sickness and loss of life