I know creatures with summoning sickness can not attack unless they have haste. My brother is using this to his defense as if you use a 3/3 creature with summoning sickness to block an attack from a 2/2 creature then the 2/2 creature would not die because you are only blocking and not attacking back.
From the rulebook:
Blocking is not "attacking back". Blocking creatures deal damage normally, whether they have summoning sickness or not. Again, the rulebook:
Yes. All blockers deal damage.
Summoning sickness only makes it so that you can not tap to activate abilities or attack with a creature the turn it comes into play.
Bonus points: You can tap a creature that comes into play the same turn if you are tapping it as a cost to activate some other ability not belonging to the creature.