The colossal task of keeping the body safe from outside and inside attacks of , virus, and other nasty creatures belongs to the immune system.
Your skin and mucous membranes are the first line of defense between invaders entering through the skin or through openings in the body.
A second line of defense exists inside the body to challenge those that make it through the first line of defense.
Sounds more like a war strategy and it is. That’s how your body treats anything that isn’t supposed to be there.
The immune system is the third line of defense. It consists of agents that target specific antigens. An antigen is that which can be identified as foreign or nonself. It can be the toxin from an insect sting, pollen, or molecules from a bacteria .
Once an antigen is noted, a response will be made that will be as rapid as possible. Sometimes it can be rather intense with high fevers,
The last part of the immune response is to remember the antigen that tried to do this. It will be ready with a much, much quicker response next time.