1) the immune system works together like a car, they help each other by using antibodies, white blood cells, the thyroids, tonsils, and even more organs. They help each other defeat foreign invaders (such a parasitic, bacterial, and fungal).
2) HIV decreases the body’s immune system dramatically and is the cause of Helper T Cells that decrease.
3) they would test for the antibodies since those are what are in vaccines, they keep the virus away!!!
Hope this helps!
Good luck!