In dental school, there's very little focus on immunology. It's a very short class. Biochem is a much larger course spanning quite a bit of material. So if you were to judge based solely on the amount of material, Biochem is harder.
That being said, I found immuno to be harder than Biochem mostly because of the teacher that taught immuno. He had a very thick accent and I found it hard to follow along. The exams were also poorly written in my opinion all of which gave the impression that the class was harder than biochem which I took at the same time. Material itself wasn't tooooo difficult to comprehend. I think with the right teacher, neither course is hard. The teacher makes all the difference haha