Diphenhydramine, prednisone, ranitidine are given to my dad before he does his cardiac cath.. is those going to help him not be allergic to iodine ?

Allergy: Iodine is not the problem. Has he ever ingested "iodized" salt? If so, he is not iodine allergic. The IV contrast used in the cath is a spontaneous mast cell degranulator that causes the allergic reaction. We suppress this with those three meds prior to IV contrast to prevent reactions.

Basically: Those meds blunt any allergic response. They are commonly given together as you describe.