Food allergies, diseases and vegan diet? What to eat?

Lauren,
Even if you are allergic to many plant foods, there are still many plant foods that you can eat, and you can get all the nutrition that you need from plant foods. I recommend you to study this page: http://www.drmcdougall.com/med_allergic.html
You can also contact a vegan nutritionist.

“It is the position of the American Dietetic Association that appropriately planned vegetarian diets, including total vegetarian or vegan diets, are healthful, nutritionally adequate, and may provide health benefits in the prevention and treatment of certain diseases. Well-planned vegetarian diets are appropriate for individuals during all stages of the life cycle, including pregnancy, lactation, infancy, childhood, and adolescence, and for athletes.” — Academy of Nutrition and Dietetics (formerly the American Dietetic Association)

I will start with some comments about health:
Milk consumption inhibits the absorption of iron:
“Secondary factors also influence the absorption of iron from ingested foods. Absorption depends upon the acidity of the stomach, the amount of ascorbic acid (vitamin C) in the foods, the kinds of amino acids present, the amount of fiber in the diet, and the form in which the iron is presented (whether heme or non-heme). Ascorbic acid is one of the main enhancers of iron absorption, and fruits and vegetables provide plentiful amounts of this vitamin. All red meats, poultry, and fish are deficient in ascorbic acid. Dairy products are not only deficient in ascorbic acid, they are also very low in iron content, and actually inhibit the absorption of iron present in other foods (for example, calcium and phosphates in milk form insoluble complexes with iron).”
Quote: http://www.drmcdougall.com/med_anemia.html

Plant Foods Have a Complete Amino Acid Composition:
“According to Dr. John A. McDougall, "any single one or combination of these plant foods provides amino acid intakes in excess of the recommended requirements...it is impossible to design an amino acid–deficient diet based on the amounts of unprocessed starches and vegetables sufficient to meet the calorie needs of humans. Furthermore, mixing foods to make a complementary amino acid composition is unnecessary.”
http://circ.ahajournals.org/content/105/25/e197.full

Why do you believe that you destroy any amino acids by cooking plant foods?
You can certainly get all the nutrition you need from plant foods. Thus, animals are harmed and killed despite that there is no need, and this is wrong. You can ask e.g. here if you want to live a life that doesn’t harm and kill animals needlessly: http://www.drmcdougall.com/forums/viewforum.php?f=22&sid=9d758ce1a0cdba79b47cf001c5e354c0

Why do people get allergies?:
“However, considerable evidence indicates that introducing certain foods (including cow's milk) too soon into an infant's diet, at the time when its intestinal tract and immune systems are still immature, may provoke responses in its tissues that will lead to symptoms of allergy later in life. Similarly, respiratory allergies andskin problems may be started at that tender age, when the infant's immune responses are unable to cope properly with the alien allergens.”
http://www.drmcdougall.com/med_allergic.html

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Some people claim we need meat if we have certain diseases, etc.

Humans have no known anatomical, physiological, or genetic adaptations to meat consumption.
We have many adaptations to plant consumption.
Vitamin C is found in plants. We can’t make it ourselves such as carnivores.
Or digestive tract is longer than carnivores, so that our food can stay in the body longer so we can digest plant matter. We need more surface area and we need more microbes.
Quote from Dr. Christina Warinner has excavated around the world, from the Maya jungles of Belize to the Himalayan mountains of Nepal, and she is pioneering the biomolecular investigation of archaeological dental calculus (tartar) to study long-term trends in human health and diet. She is a 2012 TED Fellow, and her work has been featured in Wired UK, the Observer, CNN.com, Der Freitag, and Sveriges TV. She obtained her Ph.D. from Harvard University in 2010, specializing in ancient DNA analysis and paleodietary reconstruction: https://www.youtube.com/watch?feature=player_embedded&v=BMOjVYgYaG8
So it is fallacious to assume that we would need meat for an optimal diet, or need meat if we have some certain allergies or diseases.

 It wasn't the meat, that caused us to evolve: http://www.youtube.com/watch?v=h0PF5R0ywp4
More information gathered here: http://bloganders.blogspot.se/2013/06/humans-are-herbivores-not-omnivoresmeat.html

And even if we could consume animal foods without getting health problems, so what??? Even if we had developed an "instinct for meat", so what??
The capability of having a body that can eat meat without getting a disease, does not make killing morally justified.

We can live and thrive on a vegan diet, so killing and harming animals for food in our society is completely unnecessary. The only "justification" is that it tastes good/convenience -- and pleasure is no moral justification for hurting and killing an animal. I recommend you to study this article :
http://articles.philly.com/2009-08-14/news/24986151_1_atlanta-falcons-quarterback-vick-illegal-dog-dog-fights