Relative pronouns introduce relative clauses. A relative pronoun serves to link the relative clause to the noun that the clause modifies. Some common relative pronouns are that, who, whoever, and which.
Interleukin-12 does not retard blood vessel growth itself. Instead, it causes immune cells to secrete gamma-interferon, a substance which stimulates the production of inducible protein-10.[Which introduces the relative clause that modifies the noun substance].
--Kristin Leutwyler, "An Inside Job," Scientific American (modified)
Choose a relative pronoun that is appropriate for the type of relative clause (restrictive or nonrestrictive), the type of noun being replaced or referred to (person, thing, location, or time), and the role that the replaced noun plays in the relative clause (subject or object).
Be careful about subject-verb agreement with relative pronouns.