Qbc, and grapefruit seed extract are the best as far as natural antihistamines.
Qbc, quercitin bromelain complex is derived from pineapple. Acts as a natural antihistamine, and decongestant, with out side effects.
Grapefruit seed extract is used as a decongestant, and nasal irrigation supplement. 1 drop in 1 ounce of water, mix, and put two to three drops in each nostril.
Another allergy trick is saline rinse.
Place a pinch of table salt into the palm of your hand, fill with water. Hold one nostril shut, while inhaling with the other. Gently blow. This will clear the nasal passage of excess mucus, and the allergen. Messy but effective.
Immunology is the only "cure" for allergies. Histamine is an over reactive immune response to any given substance. Allergies can develop at any time in ones life, from beginning to end, and to anything. From pollen, to food, the results are runny nose, coughing, stomach aches, scratchy throat, ear aches, and list goes on and on. Immunology is basically the hair of the dog that bit you. By injection, or by oral drops placed under the tongue in minute doses, the immune system is trained, slowly, to quit reacting to that given substance. Much in the same way as a measles, or chicken pox shot does. There is now a homeopathic line of products, specifically designed for this purpose.
I explain it in my article: https://hubpages.com/health/Is-it-a-cold-or-allerg...