In the animal world, sponges, the most basic and oldest of all animals, which have existed for more than half a billion years, possess something that probably was the first primitive immune response in animals. It is called humoral immunity. “Humor,” in this context, is an ancient Greek term that means “bodily fluids.” So humoral immunity is very tiny stuff, made out of proteins, that floats through the bodily fluids outside of the cells of an animal. These proteins hurt and kill microorganisms that have no business being there.
