The doctor explained the situation to her little brother, and asked the boy if he would be willing to give his blood to his sister. I saw him hesitate for only a moment before taking a deep breath and saying, "Yes, I'll do it if it will save Liz."
As the transfusion progressed, he lay in bed next to his sister and smiled, as we all did, seeing the color returning to her cheeks. Then his face grew pale and his smile faded. He looked up at the doctor and asked with a trembling voice, "Will I start to die right away?"
Being young, the boy had misunderstood the doctor; he thought he was going to have to give his sister all of his blood.
Source- GIVING BLOOD
P-S- not my site, since it's taken from that site so I have included the web link.