I get tons of head to head hits and contact, but no concussions?

I do American football and wrestling, and one thing I get a lot is head to head contact. Since I'm a D-lineman, I usually go against running backs who just truck, so I usually have hard head to head hits often, but I've never gotten a concussion or headaches. Same in wrestling, head bumping is common, but the most I ever had happen was just spitting up a little blood later on. I'm just wondering what is going on, do I just have a thick skull or what? most hits that would take someone out of the game and give them a concussion don't effect me. Also, I have had no iq point drops.

Any reasons?
I get tons of head to head hits and contact, but no concussions?
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