Blood Group, or the blood flow which contains aorund the body immune system helps destroy harmful things like bacteria and viruses and protects its own cells. The red blood cells create and has antigents whice varies:
1)O blood group creates A& B antigents.
2)B creates A's antibodies 
3)Ab does not create antibodies as they already have A and B

Different blood group has different effect to health conditions: 

1)Type O: Contains more natural antibodies hence stronger immune system which builds the "Auto Immune System", higher tendency to react to things.
2)Type A:Very vulnerable blood group, which falls sick often 
3)Type B: Highly Immune to viral infections, slow growing ones which affects the immune system.
4)Type AB:Has combined weakness of A and B.

Response to stress: 
1)Type O: Secrates higher level of adrenaline in response to stress, but hard time clearing off later. The only way is to excercise and reduce stress.
2)Type A: Highly tensed people, as the only way is to frequently keep in mind on how to work on meditation, excercises or walks to reduce stress.
3)Type B: Also has high response to stress, but quick reaction to meditation and relakzation.
4)Type AB: Has combination of A and B hence very highly stressed people.
Due to all this, easily get fatigue, enogh sleep, well-balanced diet and excercise will be good ways to calm these vessels down. 

Article and Picture Courtesy: CureJoy