well, basically, allergies are when your immune system goes up against something that should be harmless.
white blood cells protect against viruses parasites and bacteria, they recognise small parts on the invaders surface: antigens.

some things get recognised as dangerous when they are not; allergens.

the reaction varies depending on who many allergens are present, how many cells recognise it and who strong/fast the response is.

Allergy
there are 4 types of allergy loosely speaking as follows
type1: takes a day and gives you a mild headache and a pimple or two, most folk wouldnt even realise they had a reaction.
type2: takes about 12 hrs, gives you an ithcy skin rash then clears up
type3: can make you vomit and squirt within a few hours
type4: anaphalatic shock. full system shut down almost instantly, can kill.

although response time and severity is different, all allergy reactions involve the immune system at a cellular level.
memory cells are left to watch for re-invasion so taking the thing that made you ill again and again will only make you sicker.
you can not build up a tolerance to an allergy.

intolerance
is when your baody doesnt like something and kicks it out to be on the safe side.
intolerance does not actively involve the immune system on a cellular level and no memory cells get left on guard so you can develope a tolerance.

like chillies for example. alot of folk get ill eating chillies at first, but over time and dedicated practice the body will eventually realise that chillies are ok, infact quite good, and before you know it you carry round a bottle of encona hot sauce in your bag.


on the surface, it seems like allergy and intolerance are the same, cause both involve being ill when you take a reaction to someting you ate. but they are quite different.
said that though, most lay folk refer to any mild allergy (one that insnt anaphalactic shock) as intolerence.



ever increasing allergies
i always thought that it was farming processing and refining foods that caused the steady increase in allergies in the population.
for years i have been happy to blame farmers and politicians for making us eat crap to make them money, even though they know it's bad.
i didnt know why cultivating and refining caused reactions, but it seemed only fair to blame pesticides and fertilisers and all that.
so when i read it could be simply that we are weened at 4mnths not 6, it shocked me.
but it seems like a plausible simple and beleivable story.
mum gave me wheat 2 weeks too early now i'm allergic.
pants.
its all mums fault.
though i can blame the farmers and politcians for telling her it'd be ok!

i'm not sure that there is evidance to support the early weening = allergy theory, certainly what you say above doesnt fit.
must just be them dodgy farmers trying to shift the blame.