Type "O" blood - for example - contains anti-A and anti-B antibodies. However, it is safe to give someone with type "A" blood a transfusion of type "O" blood. Why is this?
An individual with type "O" blood will have many different types of antibodies in their plasma, including anti-A epitopes. However, the number of any one given antibody is extremely small, and overall will not have a noticeable effect.