Saliva

Saliva is the watery substance made in the mouths of humans and many animals. Saliva begins digesting food in the mouth, and moistens food to make swallowing easier. Saliva consists of 99.5% water.

Saliva collecting on the lips of a human baby. It mostly comes from salivary glands in the cheek.

The mouth is obviously the point of entry to the whole alimentary canal. It follows from this that saliva must both help digestion, and protect against infections.

The digestion part is by three enzymes, one each for carbohydrates, fats and proteins. The digestion started here carries on in the stomach. There are some other 'minor' enzymes as well, and we do not know the full story about them yet. Also there is a pain-killing opiorphin, and a wrap-around haptocorrin which keeps vitamin B12 safe from stomach acids. Finally, there are a set of anti-bacterial proteins which make life difficult for any bacteria which get in with the food.

Saliva is filtered blood, "These observations are consistent with the fact that saliva is a filtrate of blood, meaning that the contents of blood—water, salts and macromolecules—pass through capillary walls into the salivary glands." Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2741028/#:~:text=These%20observations%20are%20consistent%20with,walls%20into%20the%20salivary%20glands.