How does your body's defense system work

Your immune system uses a huge army of defender cells - different types of white blood cells. You make about 1000 million of them every day in your bone marrow. Some of these cells, called macrophages, constantly patrol your body, destroying germs as soon as they enter. This is your 'natural' or inborn immunity. But if an infection begins to take hold, your body fights back with an even more powerful defense of T- and B-cells. They give you acquired immunity so that the same germ can never make you as ill again.

Explanation:

Hope this helps :)

b) the heart is a pump made of muscle and that is difficult to grow in the lab.