Science of Food Nutrition and Health Vinod Puri (Publishers: Austin Macauley Publishers, London, 2023) Food Allergy

In the previous blog, the focus was on Food Intolerance. This blog explores allergic reactions triggered by specific foods. The term ‘Allergy’ was coined by an Australian Paediatrician Clemens von Pirquet to describe an allergic food hypersensitivity mediated by either an Immunoglobulin E (IgE) type antibody present in the lungs, skin, and mucous membranes, non-IgE mediated by T-cells, a type of white blood cell circulating in the blood vascular system, or mixed IgE/nonIgE mediated. The term hypersensitivity is used as an umbrella term covering all adverse reactions to foods and food additives, side effects of drugs, psychological reactions due to environmental factors, behavioural disorders, and others. Hypersensitivity causes reproducible symptoms or signs, initiated by exposure to a defined stimulus at a dose tolerated by normal subjects.

IgE-mediated allergic reactions are the most well-known type of food allergy, characterized by the immune system's rapid response immediately after consuming certain foods. These reactions are triggered by proteins or protein fragments in the food that are mistakenly identified as harmful pathogens, leading to the production of IgE antibodies specific to the food. These antibodies bind to basophils and mast cells in the tissues. When the food is ingested, the IgE antibodies tag it, prompting white blood cells to attack and release histamines, prostaglandins, and leukotrienes, which can cause symptoms affecting the skin, gastrointestinal tract, and respiratory system, including potentially life-threatening anaphylactic reactions.

Non-IgE allergic reactions involve cells that produce T-cells responding directly to specific food proteins, releasing mediators that trigger an inflammatory response affecting the gastrointestinal tract and skin. This type of reaction is less common and typically occurs 4-28 hours after consuming the food.

There are two types of food allergens:

Type 1 allergens consist of water-soluble low molecular weight glycoproteins that are resistant to heat, acid, and proteases, making them unaffected by cooking, storage, or digestion. Examples include casein in milk, vicillins in peanuts, and ovomucoid proteins in eggs. Cooking certain foods may reduce their allergenic properties, but it could also introduce new antigens that increase the allergenicity of proteins.
Type 2 allergens are characterized by highly heat-sensitive conformational epitopes, are easily degraded by enzymes, and are challenging to isolate. Birch pollen is an example of a Type 2 allergen that can cause itching of the skin.

Classification of Food Allergens: Food allergens are grouped under four categories:

Gastrointestinal Food Induced Allergies

·      Pollen food allergy syndrome: IgE-mediated food allergy, also known as oral allergy syndrome, is caused by cross-reacting allergens found in pollens, raw fruits, and vegetables. Examples include birch pollens (associated with apples, almonds, carrots, celery, cherries, hazelnuts, peaches, pears, and plums), grass pollens (associated with celery, melons, oranges, and potatoes), and ragweed pollens (associated with bananas, cucumbers, melons, and sunflower seeds). Symptoms typically include itching, and burning of lips, mouth, and throat, watery and itchy eyes, runny nose, and sneezing.

·      Eosinophilic esophagitis: This is an IgE-mediated food allergy attributed to the consumption of cow’s milk, meats, soy, and eggs. It occurs when eosinophils infiltrate the lining of the oesophagus, disrupting its normal function and leading to symptoms like heartburn.

·      Eosinophilic gastroenteritis: This is also an IgE-mediated food allergy characterised by the selective infiltration of eosinophils into the stomach and/or small intestine. Elevated levels of eosinophils in the blood can lead to symptoms such as heartburn, nausea, bloating, vomiting, abdominal pain or occasional diarrhoea.

·      Coeliac Disease:  Also, known as the coeliac sprue or gluten-sensitive enteropathy coeliac disease is triggered by the immune system's reaction to gluten, a protein found in wheat, barley, rye, and oats. This immune response leads to the production of antibodies that attack the intestinal lining causing damage to absorptive epithelial cells and resulting in a severe malabsorption syndrome. Symptoms include diarrhoea, bloating, weight loss, anaemia, and fatigue. Celiac disease is a hereditary condition, and its exact causes are not well understood. and is poorly understood.

·      Infantile Colic:  Some breastfed or bottle-fed infants may develop an IgE-mediated hypersensitivity. Infantile colic is a condition characterised by inconsolable crying, abdominal distention, and excessive gas production. While the exact cause of infantile colic is unknown, food allergies and intolerances are likely significant contributors.

Food Induced Cutaneous Allergies

·      Urticaria and Angioedema: Urticaria, also known as hives, occurs when histamine is released under the skin in response to certain foods like nuts, chocolates, shellfish, eggs, and wheat. This immune system reaction causes inflammation and fluid buildup, resulting in swollen, pale red rashes on the skin. Urticaria can also be caused by other allergens such as parasites, pets, dust mites, temperature changes, and chemicals. Angioedema, characterised by deep swellings under the skin, often accompanies urticaria.

·      Dermatitis Herpetiformis (DH): It is a skin condition that is caused by an immune response to the gliadin fraction of gluten. This condition is IgE-mediated, with the gluten combining with an antibody in the intestines and circulating in the blood, leading to the clogging of small blood vessels in the skin and resulting in a rash. Genetic factors, as well as certain drugs and chemicals such as iodine, non-steroidal medications, and other chemicals, are believed to play a role in the development of DH.

Food-Induced Allergies of Respiratory Systems

·      Rhinoconjunctivitis: It is a respiratory disorder that is mediated by IgE and is characterised by symptoms such as itchy eyes, redness, and tearing.  The condition is often triggered by allergens such as pollen, moulds, dust mites or specific foods.

·      Bronchial asthma: Bronchial asthma is an inflammatory lung disease that can cause breathing difficulties and is often associated with allergies triggered by immune reactions to specific foods, food additives (especially sulphites), beer, and wine.

Systemic Food Induced Allergies

·      Anaphylaxis: Anaphylaxis, also known as anaphylactic shock, is a severe allergic reaction that can occur suddenly after exposure to an allergen. Symptoms are serious and may include breathing difficulties, swelling of the tongue and throat, wheezing or a persistent cough, dizziness, or fainting. It is reported that 90% of anaphylactic reactions are triggered by common food allergens like milk, eggs, tree nuts, shellfish, wheat, and soy.

·      Exercise-Induced Anaphylaxis (EIA): Exercise-Induced Anaphylaxis (EIA) is a rare condition in which physical activity following food consumption can trigger anaphylaxis. A range of foods, including wheat, peas, beans, peanuts, corn, shellfish, certain meats and fruits, seeds, and milk, have been associated with EIA. It is important to note that anaphylaxis is not caused by food intake or exercise alone.

Factors Influencing the Incidence and Prevalence of Food Allergy

There is a significant body of evidence indicating that the prevalence of food allergies has risen over the past three decades, especially among children under the age of 5. It is believed that a common mechanism likely triggers allergic reactions, which may be influenced by genetic and/or environmental factors. While there is a strong correlation between genetic susceptibility and food allergies, scientists have yet to pinpoint a specific gene associated with this condition.

Recent research suggests that some allergies may be hereditary and linked to changes in gene expression caused by epigenetic modifications rather than alterations in DNA sequence. These modifications can occur over time due to factors such as age, environment, diet, and lifestyle choices.

Epigenetics refers to the study of changes in organisms resulting from modifications in gene expression, rather than changes to the genetic code itself. Biochemical reactions within the DNA sequence can lead to gene silencing, altering the phenotype without affecting the genotype. Epigenetic changes can occur at any point in an individual's life and have been linked to various disorders, including food allergies.

Environmental factors known to induce epigenetic changes include tobacco smoke, exposure to allergens and microbes, chemicals, maternal nutrition, caesarean birth, and stress. Research has shown that these epigenetic changes can be passed down from one generation to the next and are often reversible, neither permanent nor transient.

In Chapter 13 of my book 'The Science of Food Nutrition and Health' published by Austin Macauley Publishers, I have discussed the theoretical aspects of food allergy in detail. In my upcoming post, I will explore food poisoning resulting from different contaminants like bacteria, viruses, parasites, and toxic chemicals.

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