Imagine your body as a huge city. When your city is humming along, everything is in sync and balanced. Your organs are communicating well with each other. You are in a condition called homeostasis.

Homeostasis is what your body wants. If homeostasis is maintained, life continues. But just like a real city, life isn’t so simple. Things can go wrong, sometimes very wrong, and when they do, your immune system is your city’s first responders—your law enforcement, paramedics, EMTs, firefighters, SWAT team, and more — policing your body and ready to answer the call when there is a problem or danger, large or small.

The biological origins of your first responders are your embryonic stem cells — cells that give rise to all others in the body, such as the white blood cells that protect your city against crimes attempted or perpetrated by infections and foreign invaders.

Your liver metabolizes the nutrients needed to provide these first responders with just the right amount of energy and vitamins to ensure good health. Your biomes, which naturally inform the immune system what is friend or foe and work together to strengthen it, are pro[1]viding the first responders all the support they need to be efficient when unwanted parasites, bacteria, and viruses appear.

Some embryonic stem cells begin their training as first responders before birth in your bone marrow (B cells), and others in glands like the thymus (T cells) or organs like the lung and liver (monocytes).

Dr. Bob is the author of IMMUNITY STRONG: Boost Your Body's Natural Healing Power and Live to 100.