When your body is healthy, you have a powerful army that defends you against invading bugs and foreign particles, such as viruses, bacteria, and parasites. That army is your immune system, and it polices your gut and blood stream for anything it deems foreign. Your immune system is a complex network of specialized cells and organs that work hard to differentiate between self and “nonself”—that is, between what’s you and what’s not.
Your immune system’s main weapon is to make antibodies against germs and large particles. Unfortunately, your army can sometimes mistakenly identify your own cells as foreign.