Is it an allergy, or an intolerance?

[Note: this was originally part of the post titled Allergy: I do not think that word means what you think it means. I've moved it into a separate section to make the original post more manageable.]

When it comes to abnormal or unusual adverse reactions to food, most resources divide them into two categories: allergy and intolerance. There are also adverse reactions related to an element of the food itself, which may not really fit into either category. Some types of food poisoning, for example, have allergy-like symptoms.

Differentiating between different types of adverse food reactions can be complicated.

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The most commonly-known group of food intolerances is the type which involves difficulty or inability to break down certain food elements. Often this is because of an inherited missing enzyme. Lactose intolerance is probably the mildest and most common form of this. Other metabolic disorders that cause food intolerances can be much more serious and difficult to manage, such as Congenital Sucrase-Isomaltase Deficiency (CSID) and Phenylketonuria (PKU).

Many resources simplify allergy vs. non-allergy by saying that an allergy involves the immune system, while an intolerance does not. But I'm learning that this is not necessarily true. Some conditions, such as celiac disease, are triggered by food and involve an abnormal immune response, but are not allergies. There are also conditions such as eosinophilic disorders which damage cells or trigger production of abnormal cells by a person's body. These can be triggered by specific foods, but allergies may or may not be detectable.

I've often thought of allergy vs. non-allergy relating to whether the reaction involves histamine release or not. After all, allergy medications are antihistamines, right? An allergic reaction must be any adverse reaction that involves histamine, I thought.

Not so. There are actually a number of adverse food reactions that look and act like allergies, and involve histamine, but aren't actually allergies. Some of these, like scromboid fish poisoning or histamine intolerance, are from ingesting histamine in food rather than producing it in the body. Others, like mastocytosis or direct histamine liberation, may involve the overproduction of mast cells or the release of histamine within the body caused by something other than an allergic reaction.

Even the presence or absence of particular symptoms can't firmly differentiate between allergic and non-allergic reactions. Typical allergy symptoms such as hives, asthma, itching, swelling, changes in blood pressure, vomiting and diarrhea can all be caused by other triggers.

One might think that, at least, symptoms of anaphylaxis from consuming a food would be a clear indicator of allergy. I would have thought that if a person had negative allergy tests but got hives, wheezing, itching, and a life-threatening drop in blood pressure or difficulty breathing on food challenge, that would be considered proof that the test was wrong. Some allergists consider an anaphylactic-type reaction to be final proof of an allergy, but others may not.

I've recently learned that there's actually something called an anaphylactoid or pseudo-allergic reaction. It looks and acts exactly like an anaphylactic reaction. Same symptoms, same treatment. Both are life-threatening. The difference is that anaphylactic reactions include the presence of IgE antibodies, and anaphylactoid reactions don't. Anaphylactoid reactions are most common as reactions to non-food substances such as medications and contrast mediums, but anaphylactoid food reactions have been reported.

The more I learn, the less I'm sure I understand. Perspectives and approaches vary among doctors and researchers, which complicates matters even further.

What seems clear is that both IgE antibodies and a specific set of typical allergy symptoms need to be present for unanimous agreement that an allergy exists. You can read a bit more discussion of that as it relates to Baby E's situation here.