How do you do?

There is a difference between 'How are you?' and 'How do you do?'

When you meet someone, you may say 'How are you?' if you have not seen him for a long time. He should then say 'I'm fine. Thank you.'

However, when you are introduced to someone whom you have not met before, you should say 'How do you do?'

What do you think he should answer? Well, he should repeat what you have uttered, that is, 'How do you do?'