i'm ab+ i'm pretty easy going, hard to get my blood boiling, except in polotics so if ever i start a rant tell me to shut up lol anyways the reason they had you put your blood type on your helmet is for the same reason they have it on dog tags for the troops, if you need blood or can give blood it makes it easier. the compatabilities of the blood types are as followed;
O- can give blood to anyone, but can only receive from another O- also known as the universal donor
O+ can give blood to A+ B+ AB+ and of course other O+
A- can give blood to A+ AB+ A- and can get blood from O- and A-
A+ can give to A+ AB+ and get from A+ A- O+ and O-
B- can give to B- B+ AB+ and get from B- and O-
AB- can give to AB+ or AB- and get blood from other AB- A- B- or O-
AB+ is known as the universal recipiant and can get blood from anyone and can only give to other AB+
bad things happen when the wrong person gets the wrong blood so when hospitols don't know a person's blood type and they need a transfusion they give them O- until they know the blood type, i dunno about the predict your personality but hey i could happen