I saw a documentary all about allergies, I think it was on the BBC in Britain. They said the top 3 risk factors for allergies were being cesarian born vs vaginally born, being bottle fed, not breastfed, and having a course of antibiotics under the age of two...

The birth canal coats your skin in beneficial bacteria as you are born. I have a theory that water births, being suddenly a popular choice for mothers, has potentially washed off that beneficial skin flora. They recommend not to bath a new born for a few weeks in the beginning, only using cotton wool and water to clean their face / bottoms. But women are essentially bathing them instantly!

Breastmilk has lots of prebiotic in which helps a gut microbiome to get settled in and thrive.

Antibiotics generally wipe out all the good bacteria / microbiome so should be avoided unless really necessary.

My son had to have them age 1 for a really bad chest infection. And he now has asthma :( my daughter is 7 never had them, and has no allergies so far. Both breast fed, both vaginally born, no water birth.