In Mount and Blade Warband you can get helmets that add defense to your head (as you would probably guess) but, what I am wondering, is if getting hit in the head with and arrow, bolt, throwing axe, etc. with a helmet with high defense causes it to do less damage than if I had lower head defense and higher body and leg defense.
All defense is independent in M&B Warband. Helmets only protect your head, body armor only protects body, etc. There are a few exceptions, but these are specific pieces of body armor that affect legs as well. Besides that, the general rule is that your armor will only affect the specific body part it goes on.