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Allergens and haptens : what is the difference and connection between them?


Allergens are certain substances, upon contact with which a person has an allergic reaction. These substances are also called antigens. They are generally relatively small proteins with molecular sizes less than 70 kDa . In addition to proteins, allergens can be protein-polysaccharide and protein-lipid complexes, certain non-protein substances, chemicals, immunopreparations , and so on.

Before a person begins to feel certain symptoms of the disease, he can calmly come into contact with these substances for years, or even his whole life. Someone will not develop an overreaction to a particular stimulus at all, and someone may one day encounter new symptoms.

A specific antigen will turn into an allergen for a specific person only after at some point, due to certain circumstances, the person becomes sensitive to it. This phenomenon is also called sensitization.

However, sometimes a patient with an allergy may not be sensitized to a particular allergen, but will still receive a certain allergic reaction. In addition, some antigens, such as peanuts, cause a reaction on their own, others, in particular penicillin, only after certain processes in the body.

The specific mechanism depends on which type of allergen affects the person.

Substances that independently trigger an allergic reaction are also called complete or true allergens. And those who need “outside help” for this – incomplete allergens or haptens .

Substances of each of the two types interact in their own way with antibodies.

Complete allergens include:



pet dander

fungal spores.

These allergens are sometimes referred to as class I allergens. If they cause sensitization by ingestion, then it is believed that the reaction of the body can be quite strong with generalized symptoms and the risk of anaphylaxis. For example, peanuts, wheat, milk and other foods have such properties.

Incomplete allergens are considered




some fruits,


These substances acquire their allergenic properties only after they form a complex with the proteins of human tissues.

More precisely, haptens stimulate the body to produce antibodies and thus an allergic reaction only when they are bound to a larger molecule called a carrier molecule.


Even the term ” hapten ” comes from the Greek haptein , which means “to fasten” .

The hapten-carrier complex stimulates the production of antibodies and becomes immunogenic , that is, capable of inducing an immune response. But an unbound hapten is not able to provoke it.

The bound hapten specifically reacts with antibodies formed against it to elicit an immune or allergic response. Thus, although the hapten cannot elicit an antibody response on its own, it is able to bind to antibodies and act as an antigen.

Skin (contact) allergy is the most common form of immunotoxicity in humans, caused by haptens that penetrate the stratum corneum and modify endogenous proteins.

Many drugs that cause allergic reactions, such as penicillin, also act as haptens . So, when injected or ingested, penicillin reacts chemically with proteins in the body, forming a hapten -carrier complex . This can lead to a life-threatening syndrome called anaphylaxis . Other haptens may be synthetics such as benzolarsonate , benzenesulfonate , or trinitrophenol, or natural polysaccharides such as lactose.

Researchers have used haptens to create synthetic vaccines to immunize people against various infectious organisms.

Interestingly, penicillin and its relatives are spontaneously converted to the active form, while many other haptens are deliberately modified in the laboratory to make them “reactive”.


In addition, it was found that incomplete allergens, or as they are also called class II allergens or non-sensitizing elicitors , have homology with aeroallergens .

Sensitization to them occurs indirectly to cross-reactive aeroallergens through the respiratory tract. Class II allergens are thought to be associated with milder local symptoms such as oral allergy syndrome (OSA).

A well-known example of such a non-sensitizing The elicitor (incomplete allergen) is Mal d 1. This apple protein is a homologue of the group 1 birch allergen Bet v 1. Inhalation of birch pollen induces IgE antibodies to Bet v 1, some of which are crossover.

Ingestion of apple does not result in the production of IgE antibodies , but may induce the activation of mast cells loaded with pre-existing (birch-induced) cross-reactive IgE antibodies . In this case, birch pollen completely inhibits IgE binding to apple proteins, while apple gives only partial inhibition of IgE binding to birch allergen.

Clearly, oral exposure to Mal d 1 is unlikely, if at all, to cause the immune system to produce IgE antibodies , whereas inhalation of the very similar Bet v 1 protein, in not much different amounts, often does.

Scientists believe that Mal d 1 does not have the properties necessary to call this protein a food allergen. One structural feature that may explain part of the difference between these defective food allergens and true food allergens such as peanut allergens is susceptibility to low gastric pH and proteolytic enzymes, particularly pepsin. That is, unlike real food allergens, Mal d 1 is easily decomposed by temperatures and digestive enzymes.

Indeed, in practice, the high antigenicity of fresh fruits and vegetables is well known in comparison with thermally processed vegetable products. It can also be explained by the instability of an inferior food allergen.

Although latex fruit syndrome is considered a class 2 food allergy, it should be remembered that in addition to OSA, latex, which is a real allergen, sometimes provokes generalized symptoms. Cross-reactive food allergens associated with this syndrome, such as class I chitinase , are somewhat stable and can reach the gut without being completely destroyed.

Another potential danger of haptens is that these substances can change the structure of the carrier cell to which they are attached. This can lead to the fact that the body perceives its own cells as foreign. This can lead to the development of autoimmune diseases, in which the body produces antibodies against its own cells. Notable examples of autoimmune diseases are rheumatoid arthritis and systemic lupus erythematosus.

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