Where does saliva come from?

Saliva is made in the same way that you make tears.

You have glands, which have a rich blood supply. When the blood goes through it passes through very thin-walled capillaries, which are a little bit leaky. It's exactly the same as if you go to the garden centre and you see those watering systems where the pipes have small holes in them. You turn the tap on and the water going through squeezes some of the water out of the pipe.

Why do you make saliva, in other words watery liquid and not blood? The answer is the walls of these tiny capillaries are just big enough for water molecules but they're too small to let the bigger proteins and cellular components of blood leach out. Some things can get out: the water, and some antibodies but that's about it. 

Saliva also stops viruses because it contains antibodies. It's got an antibody called IgA which is one of the immunoglobulins which can neutralise viruses. Yes you can mop up a lot of colds and viruses and bacteria with the constituents of saliva. The antibody is secreted - you actually have a pump which pushes them into the saliva. They're added after you've made the saliva or tears.