I see there is no way to cure eczema, but how much do antihistamines like claritin, (loratadine) clarinex, or zyrtec help prevent eczema?

Eczema: Eczema starts with itch so if you can prevent itch with these antihistamines you can also prevent the development of eczema.

Eczema - cure: Eczema can go away if it is caused by something like a food or chemical but it requires a thorough investigation by a allergist or dermatologist. Some hereditary forms cannot be cured but only controlled by medications. Antihistamines help to reduce itching but cannot cure this condition.