When one organism lives inside the body of another, and both benefit, the relationship is called

I believe it is called symbiosis

The term for this is Symbiosis, Symbiosis is achieved when two organisms benefit from each other.

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products