Against advice, when my infant son gets small bumps I often jump up to hold and comfort him while he bursts into tears. Is it true that I shouldn't react with this behaviour?

I would believe it's true. Kids will react/behave to how the parents react. If you immediately jump up and run to comfort him,he's getting some attention. If it's small little bump , you could be encouraging over-dependence and indulging him. Some kids will actually fake being hurt for attention if they've learned this technique works. I will take a second to access the situation, if it's a small bump, I would let him learn to comfort himself a bit first and assure him that it is ok, please be more careful next time and encourage him to pick himself up. It's tough, but you and your child can benefit from it in the long run.