What is a blood feud?

When Mr. Burns falls ill and desperately needs a blood transfusion, Homer discovers Bart has Mr. Burns' rare blood type, and Homer urges his son to donate, promising that they will be handsomely rewarded. However, Mr. Burns knows nothing of this promise, or even of the Simpsons' desire for a reward, and having received the blood, all he does is send the family a card. Enraged, Homer writes an insulting reply, but Marge convinces him at the last minute not to send it. Later the letter goes missing, as Bart has mailed it, still thinking its message is called for.