Immune system. This is a very in-depth topic that could never be described with adequate detail in this little box. Please check out this website for specific information on the entire immune system. http://www.niaid.nih.gov/topics/immunesystem/pages/default.aspx.
Very long answer. Check on google. This is a very complicated subject. It is good for at least one book.
Controls microbes. The specific and non-specific immune systems work together to maintain a balance with the microbial world. Bacteria and fungi that colonize the body's surfaces; the skin, the respiratory tract and the GI tract are met at those surfaces they do not invade. The micro biome (these colonizing microbes) play an important role in homeostasis. Read more...