That reminds of the quote from General Patton when he said,"It's good that you men are brave, and willing to die for your country, but the object of war is to make the enemy soldier die for his."
Same thing, isn't it? The object is to hit the other guy and to see to it that he doesn't hit you. The object is to see if your opponent can take a punch, not to see if you can take his.
If you are asking if somebody who has ring experience can become invulnerable to punches, well:
Joe Louis got knocked out, didn't he?
Or what about MIke Tyson? OR, better yet, I love that quote from Lennox Louis when somebody asked him about the time that he was knocked out, and his response was,"If you had gotten hit with that punch, you would have been knocked out too!"
My understanding at this point is,anybody, no matter who they are or what their background or level of experience, if they get hit hard enough in the right spot, they're going down!
I think the only thing you may have to worry about is any lingering damage from repeated blunt trauma to your head. That said, most times people spar in controlled levels of contact. When you spar, it is with the goal of sharpening your skills in mind, and not with a mindset of hurting your training partners. Even the best boxers or muay thai practitioners don't go "all out" when sparring in the gym. The risk of injury would set them too far back in the professional realm.
Sparring can get you acclaimated to getting punched, to a point. This isn't to say that you're going to eat punches from heavyweight boxers, but as was stated above it will help train a lot of inately 'bad' fighting habits out of you like closing your eyes when you get punched, turning away from your opponent, holding your breathe while fighting, etc. It is in no way to make you immune to getting hit. If that was the case, you can run into any wall face-first and accomplish the same thing without a sparring partner.
Chin comes with experience. I've been in a few fights and I've done alot of sparring, so I've developed an ability to tolerate the strikes I'm not very good at blocking/countering. It shores up my weakness on that front.
The biggest issue that alot of guys have is with how they take jabs, because you can't really train your chin to take a huge hook or a knee very easily. If you learn to roll off of jabs and to absorb those blows without getting stunned, then you have "good" chin.
In order to take that to the next level, you have to have alot of experience. Guys like Mark Hunt have been fighting for years developing thier chins. I don't know if theres a secret to being able to take that many shots, but I'm working on it.
Sparring, sparring and more sparring. Let me say it now: Nothing will make you immune to punches. If you get hit and you don't end up hurt, either you're lucky or your opponent didn't hit you right. Sparring only makes it so that you don't automatically go into panic mode the second that you do get hit. That's it.
I could go into a whole bunch of stuff like rolling your chin into your shoulder, making sure you're breathing out when you punch so your chest isn't full of air when you get popped in the gut. But the truth is this: If you don't train enough to get those things down to where they're second nature, you'll end up thinking about it after you get hit and it won't do you any good.