Though on the rise, food allergies are fairly rare. If you have one, particularly a serious one, you may wonder why you got so lucky. While I can’t answer for the grand plans of the universe, I can provide a few insights on the subject. In this post I will be discussing the factors that make some proteins more likely to cause allergies and some of us more susceptible to them.
Do all proteins cause allergies?
Proteins are necessary for living cells to survive. They are ubiquitous and diverse with nearly 13,000 different protein families. Despite all that variety, just 0.16% of those families cause 80% of food allergies. Clearly, there is more to being an allergen than just being a protein.
What factors influence the potential of a protein to cause an allergic reaction?
1. Characteristics and structure of the protein
2. Amount of exposure to the protein
3. Genetic susceptibility of the person exposed
What types of proteins are more likely to cause allergic reactions?
Proteins provide countless essential roles in living organisms from balancing fluids to orchestrating chemical reactions. A protein’s function impacts its allergenicity. The majority of food allergies are caused by proteins involved in protection or storage.
Why protection and storage?
Protection proteins do exactly that. They protect plants from invasion by pests or pathogens. They are usually found in higher concentrations in seeds and tubers. These proteins are tough since they are designed to function even in hostile environments. Their ability to withstand aggressive atmospheres makes them perfect candidates for resistance to cooking and digestion. Profilins and Bet v 1, the main allergen in birch, are among defense proteins.
Storage proteins are found in seeds such as grains, nuts, and legumes. Their main purpose is to supply the nutrients required for the sprouting and developing of seedlings. There are two primary issues with these proteins. They are highly concentrated in seeds leading to greater exposure, and many of them are not damaged by heat. For example, globulins, which comprise over 50% of the protein in some species of nuts, are stable up to 200 degrees Fahrenheit. Albumins, like those found in soybeans and peanuts, are another family of storage proteins implicated in a large number of allergic reactions.
Beyond function, are there other protein properties that enhance allergenicity?
In order for a protein to initiate an allergic response, the allergenic component must be at least 20-25 amino acid residues long. Otherwise, it will be too short to bind properly to antibodies and the IgE system won’t respond.
Allergenicity is also dependent on stability. Sensitization through the gut, which is typical of many allergens like those in nuts, requires the protein to survive a number of onslaughts. It has to withstand cooking temperatures. It has to remain intact through the acidic conditions of the stomach. Additionally, it has to endure many enzymes and bile salts bombarding it throughout the digestive process. Most proteins are not up to the task. Some manage all too adeptly though.
Proteins make 3D structures by folding in and crosslinking. These arrangements look a bit like noodle blobs. As Giada De Laurentiis would tell you, not all noodles are created equal. Some protein configurations hold up better to cooking and digestion. For instance, beta structures are more resilient than alpha helices. Other structural components like links between different regions through disulfide bonds increase a protein’s hardiness. Plus, size does matter; smaller proteins are more able to withstand the heat of cooking.
Are certain protein sources more likely to cause allergies?
Immune responses, including allergies, are based on the body’s ability to distinguish between self and non-self. If a food protein is similar to our own proteins, it’s less likely our immune system will recognize it as different. Animal proteins, particularly from mammals, are much less prone to trigger allergic reactions for this reason.
Remarkably, a protein’s source may matter in other ways. Soy allergens usually elicit much milder reactions at much higher doses than many other allergenic foods. Research has suggested this might be due to the anti-inflammatory phytochemicals present in soy dampening the immune response to this ingredient.
What traits make a person more likely to develop allergies?
Some scientific studies indicate the risk of developing allergies is about 80% due to genetics. Inherited programming causes some immune systems to be unnecessarily sensitive. A genetic predisposition for an overly reactive immune system is known as atopy. Atopic diseases include eczema, asthma, pollen allergies and food allergies. If you have one parent with atopic disease, you have a 25% chance of also developing one. Those odds jump to 50% if both your parents have atopy.
The inherited immune system anomalies associated with allergies may have broader impact too. Research has shown the same genetic factors that increase vulnerability to allergies augment susceptibility to inflammatory and autoimmune disorders like Crohn’s disease and psoriasis.
That’s nature, but what about nurture?
Although genes clearly influence allergies, environment is decidedly also involved. Humankind’s genes haven’t changed in the last 50 years, but the incidence of atopic disease certainly has. In developed countries, the rates of asthma and allergies increased significantly in the last half of the 20th century. For instance, in Australia, pollen allergies rose from 22.5% to 44% between 1982 and 1997.
One theory behind this surge is called the hygiene hypothesis. It speculates that modern environments have become too clean and our immune systems too sheltered from infections. Research has confirmed this hypothesis has merit, but no definitive conclusions can yet be made.
For my final part in this series, I will review how food allergies are diagnosed and treated. Come back to discover how to distinguish between an allergy that’s intolerable and an intolerance.
References:
1. Mills and Shewry. (2004). Plant Food Allergens. Blackwell Publishing.
2. Marenholz et al. (2017). Genome-Wide Association Study Identifies the SERPINB Gene Cluster as a Susceptibility Locus for Food Allergy. Nature Communications.
3. Masilamani et al. (2012). Determinants of Food Allergy. Immunology and Allergy Clinics of North America, 32(1), 11-33.
4. Thomsen. (2015). Epidemiology and Natural History of Atopic Diseases. European Clinical Respiratory Journal.