Two friends, Mike and Jake were returning late at night from a club in the same car. Suddenly, Mike, who was driving, lost control of the car and...

Two friends, Mike and Jake were returning late at night from a club in the same car. Suddenly, Mike, who was driving, lost control of the car and they crashed into a big tree. Mike ended up with only a few bruises and scratches, but Jake got hurt. 

Jake had internal bleeding and was taken to a hospital, but he lost a lot of blood and needed a blood transfusion. Mike, feeling responsible for his friend's condition, wanted to donate blood to his friend. Both Mike and Jake had blood type O. So, in the ER, Mike gave 2 pints of blood to Jake. 

Two weeks later, Jake became anemic. So, Mike, being a good friend, decided to give blood again. However, after the second transfusion, Jake went into an anaphylactic shock (severe immune response) and almost died. 

What, do you think, was the problem here?