Lysozyme

The second protein to have its structure determined, after myoglobin, by X-ray crystallography, in 1965.

Breaks up peptidoglycan present in the bacterial cell wall, which is thicker in Gram-positive bacteria, which is what this enzyme seems to target.

Part of the inate immune system.

It is present on basically everything that mammals and birds excrete, and it kills bacteria, both of which are reasons why it was discovered relatively early on.