Worldwide allergies are steadily increasing.

In June 2022 we observe World Allergy Week.  World Allergy Week is an annual initiative of the World Allergy Organization (WAO), together with its Member Societies, to raise awareness of allergic disease and related disorders and advocate for the provision of training and resources in the diagnosis, management, and prevention of these diseases and asthma, which are rising in prevalence around the world.

Nowadays, an allergy is defined as a specific hypersensitivity involving the immune system, where the body’s immune system has a reaction to a substance it sees as harmful which are actually harmless.  Internationally allergies are classified into 5 types.

Immediate or Type I allergy

  • The most frequent allergic reactions are type I reactions.
  • Type I reactions are instant reactions and symptoms appear after a few seconds to minutes.
  • It includes anaphylactic reactions (the most severe form of allergic reaction). Anaphylaxis is a medical emergency that can lead to sudden, life-threatening respiratory failure.  Symptoms include difficulty breathing, swelling, low blood pressure, bluish skin, and shock.
  • Type I reactions take place primarily on epithelial surfaces (skin, lungs, gastro-intestinal tract).
  • The immune system produces specific immunoglobulin E or IgE antibodies in response to allergens such as pollen, animal dander, insect bites, dust mites, or certain foods.
  • Type I reactions are the classic causes of allergic rhinitis, asthma and atopic dermatitis.

Cytotoxic or Type II allergy

  • Symptoms appear after minutes to hours.
  • Type II allergies are characterized by the formation of immunoglobulin G or M, with reactions acting against cell surfaces.
  • IgG and IgM antibodies damage cells by activating the complement system of the immune system such as in:
    • Autoimmune haemolytic anaemia
    • Immune thrombocytopenia
    • Autoimmune neutropenia
    • Goodpasture syndrome
    • Graves’ disease
    • Myasthenia gravis

Immune complex-mediated or Type III allergy

  • Symptoms set in after several hours.
  • Type III allergies are characterized by the formation of immunoglobulin G or M, with reactions against soluble antigens.
  • In type III reactions, IgM and IgG antibodies react with allergens to form immunocomplexes (antigen-antibody complexes) such as in:
    • Exogenic allergic alveolitis
    • Lupus
    • Serum sickness
    • Arthus reaction
  • Furthermore, IgG antibodies against food stuff are suspect to mediate delayed food allergies.

Delayed hypersensitivity or Type IV allergy

  • Symptoms set in hours to days later.
  • Instead of antibodies, white blood cells called T cells or lymphocytes control type 4 hypersensitivity reactions.
  • The three subsets of type 4 hypersensitivity are:
    • Contact dermatitis
    • Tuberculin-type hypersensitivity
    • Granulomatous-type hypersensitivity
  • Some common causes of type 4 hypersensitivity reactions include exposure to poison ivy, certain metals, and medication such as antibiotics or anticonvulsants.

Type V allergy

  • Type V allergic reaction is a relatively newly described reaction in which an antibody targets receptors on the cell surface that are normally activated by hormones.
  • It is likely a subset of type II hypersensitivity reactions, as it involves an antibody targeting a specific structure within the body, leading to the clinical syndrome through abnormal cell signalling, either through the antibody activating a receptor or through blocking the binding of the normal hormone.
  • No dermatological type V hypersensitivity reactions have been described.
  • Common non-cutaneous examples of type V hypersensitivity reaction include:
    • Graves disease
    • Myasthenia gravis

How is a food allergy different to food intolerance?

Intolerance refers to an individual’s ability to handle different types of food or drink.  Food intolerance is not a true allergy.  There is no clearly defined immune response and the symptoms that often develop only affect the digestive tract.

An intolerance is normally found in the context of a deficiency in certain enzymes that break down the components of food.  The absence of these enzymes results in abnormal by-products that produce the symptoms of intolerance.

Food intolerance means the individual can eat small amounts of food without obvious consequences while eating larger amounts may lead to stomach cramps, nausea, or diarrhoea.

In contrast, a true food allergy tends to affect multiple organ systems within the body and tends to have a poorer outcome.  For example, a true food allergy means that the individual is required to avoid that food for life or risk the consequences.

A good example of the difference between a food allergy and food intolerance is comparing coeliac disease, an allergy, with lactose intolerance.  Patients with coeliac disease have an allergy to gluten.  In coeliac disease, the ingestion of gluten results in a specific immune response against the gluten, leading to inflammation and the destruction of the lining of the small intestine.  This leads to the malabsorption of important vitamins and minerals, which if left untreated can be life-threatening.  Lactose intolerance is due to a deficiency in the enzyme (lactase) that breaks down lactose.  The absence of this enzyme results in abnormal levels of lactose in the bowel, that leads to the feeling of bloating, diarrhoea, cramps, and nausea.  While uncomfortable, the continued ingestion of lactose is not life-threatening.