I have several blood donor cards.

In America, when you suggest to give your blood donor card, does it mean you will give your blood allocated on that card? I have donated blood before in Korea, but the system in the states seems to be different.

M : Hello, Linda. How are you?
W: Hi, Brad. I’m fine. How about you?
M: I’m pretty good. What’s up?
W: You remember my cousin James, don’t you?
M: Sure. We had a great time at the amusement park last year. How is he doing? W: Not well, unfortunately. He’s in the hospital.
M: Really? What happened to him?
W: He had a car accident this morning, and he needs blood donations.
M: Then, maybe I can help. I have several blood donor cards.
W: That’s why I’m calling. Can you give me the cards for him?
M: Yes, of course.
W: Thanks a lot.

It doesn't make a lot of sense to me. If you donate blood, they do give you a card. In times of scarcity this is supposed to guarantee you access to blood from the blood bank ahead of someone who wasn't a donor.

But it's not transferable to other people that I am aware of. I am not sure what collecting cards from other people would do.

If you need 3 pints of blood during some surgery and you have only donated 2 pints, it's not like they are going to refuse to give you what you need.

Are you sure this is American?

When you need life-saving measures, you get life-saving measures. You don't, as Dave said, not get needed blood because you yourself are not a donor.

I have a blood donor card, which simply facilitates the process of giving blood. They already know who I am, where I live, etc.

Blood donation is of course entirely voluntary.