15 years ago I listened to a radio interview with Australian scientists from the Murdoch Children's Research Institute on this issue. They explained how to safely introduce these foods to babies. I took these steps with both my kids at 6 months. They are now 13 and 9 and have always been allergy free. The only food that babies should never have is honey because of the risk of botulism. After 12 months, honey is safe.
1. Put a very small amount of peanut butter on the outside of the baby's cheek. Wait to see if there is reaction.
2. Repeat next day.
3. Next day, put a very small amount of peanut butter on their tongue. Wait to see if there is a reaction.
4. Repeat next day.
5. At this stage you should be pretty confident that peanuts are safe.
6. Repeat with other foods, eg egg, strawberries.
These days the advice is not to put the food on the baby's skin and instead to put it in a diluted form on the inside of the baby's lip.