It is now believed by the science community, (and me too), that some allergies can be prevented to some degree, so those of you out there that don't have kids yet, will want to read this section carefully ;-)
Nowadays, we live in a pretty polluted world, which tends to overwhelm our systems, if we're not programmed to accept these allergens. Along with the allergens that most people think of (dust, dust mites, pollen, etc), we also have to consider cigarette smoke, exhaust fumes, etc (air pollution.) And, we have lots of chemicals now in our home and work environment that we're not designed to endure (like formaldehyde fumes from the building materials in our homes.)
On the other hand, at the same time, we are creating excessively clean environments in our homes. Imagine all the products that have become available in the past 10-20 years to help us keep our homes clean and comfortable and more STERILE. Included are antibacterial products that keep germs at bay, innovations like high quality vacuums that keep our carpeting much cleaner than years ago. Almost everyone has a dish washer, that completely sterilizes the eating utensils. The list does truly go on and on.
And, our behaviors have changed. Gone are the days where we'd let our kids run around in the yard barefoot. Parents freak out when their babies pick toys or dropped food items off the floor in the kitchen, and stick them into their mouths. Pacifiers are constantly washed when they fall on the floor. We go nuts when the babies try to eat sand. Our kids spend much more time inside this excessively clean home than they did 30 years ago, playing computer, watching TV, etc.
The trouble with our clean behaviors, and keeping our homes so clean, is that this new generation of children don't have regular exposure to the natural, harmless, germs and allergens that are supposed to be accepted by our bodies. Studies have shown that children raised in homes with 2 or more dog or cat pets don't have as high an incidence of pet allergies that children have that have been raised (as babies or very young children) without dog or cat pets. Did you know that families that live in homes with more than two toilets tend to have more allergy trouble because these toilets stay cleaner without everyone in the house using one? Our kid's bodies do actually learn to accept the germs from the rest of the family.
Our kid's bodies are supposed to LEARN how to handle these foreign things. By depriving their little bodies of these allergens, we are actually creating allergies. So, in order for them to do this, we do need to expose our babies and children to these items. If we want our kids to grow up without allergies, we need to stop being so overly protective about cleanliness and allergens. We need to gently expose our kids to normal, everyday germs and environmental allergens. This should be gentle exposure.