Answer:
Opportunity cost.
Explanation:
When economists refer to the “opportunity cost” of a resource, they mean the value of the next-highest-valued alternative use of that resource. In economics, a trade-off is defined as an "opportunity cost." For example, you might take a day off work to go to a concert, gaining the opportunity of seeing your favorite band, while losing a day's wages as the cost for that opportunity
I would start by stating that it's the white blood cells that are trying to fight you. Mention phagocytosis. And describe what it looks like. Basically this is just asking for you to explain how the immune system works within the body. You can PM for a good site to use as a source if you don't know how this works within the body.
More specifically, cardiorespiratory endurance is the level at which your heart, your lungs, and your muscles work together. When we exercise, your muscles need fuel, one of them being oxygen. And it is your heart, lungs and blood vessels that have to provide this to your muscles.