The immune system is probably one of the most important bodily functions, serving as the defense against all infection that can damage the body and mitigating the damage. This happens via four different types of cells that do different things
Leukocytes are basically the alarm for the immune system. It tells the rest of the immune system when something is amiss, and sends in the next line of defense directly afterwards.
Phagocytes are the first line of defense for the immune system; their job is to start consuming the cells right away. The T Cells then receive signals from the Phagocytes to send in the second line of defense, which causes the symptoms of infections and viruses that we know of, and continue to attack the invaders.
Lastly, we have the B Cells. The goal of the B cells is to take information from the infection, and work on making antibodies that will help fight off the same infection again later, should you catch it again. Antibodies are also what helps you build complete immunity to certain infections, like chickenpox, and they are also exploited to make vaccines.