I see there is no way to cure eczema, but how much do antihistamines like claritin, (loratadine) clarinex, or zyrtec help prevent 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.
Eczema. Eczema starts with itch so if you can prevent itch with these antihistamines you can also prevent the development of eczema.