TL;DR: -ab is related to antibody, -ib is related to inhibitors.
these are immunotherapy drugs, ib means inhibitor and ab means antibody IIRC.
Drugs that end in mab are monoclonal anti-bodies. They are cloned copies of anti-bodies from various sources that target something in the body.
Caution: etymologically-oriented answers generally miss important nominological / nymological / onomatological / onomastic foundations. This namespace is a "synthetic" social construction. It has interesting "quirks", "gotchas", and "zingers".