The domino effect
Sometimes we are sensitised by one selected animal and sometimes by several of them. The building proteins of the individual allergens are responsible for this hypersensitivity [1]. We group allergenic proteins of plants or animals with a similar structural structure into families. It is this similarity of proteins that can trigger the immune system and result in a situation where a person with an allergy to a dog reacts to similar proteins from the same family found in other mammals, such as cats.
An allergy to one group of animals can therefore lead to a reaction on contact with other animals. In this case, we speak of allergy by cross-reactivity [2].
We know more and more about the allergenic proteins of individual animals. We can also determine E (IgE) antibodies in the blood specific to the allergenic proteins. This allows us to predict whether the allergy will be confined to one species or whether it will involve a larger number of animals.
Which proteins are responsible for multi-animal allergy symptoms?
Lipocalins
The cat, dog and horse proteins that play a particular role in allergies to these animals are lipocalins. They are found in the saliva and epidermis of every furry animal. Therefore, sensitisation to lipocalins is associated with a high probability of allergic reactions after contact with different fur animals due to cross-reactions [1]. Significant concentrations of allergens will be found in the bedding and on objects in the rooms where the animals are kept.
Lipocalins are lightweight proteins, which makes them stick to human clothing and hair. They can therefore also be carried into places where animals are not normally present, such as an office or school [3]. Researchers have shown that their concentration in such places, despite the absence of animals, sometimes reaches levels sufficient to trigger an allergy. Moreover, it can even stimulate asthma exacerbations, especially in children [1]. It has also been shown that individuals sensitised to higher numbers of lipocalins experience more severe symptoms and more frequent asthma exacerbations [7,9].
Almost all lipocalins are inhalant allergens. The exception is cow’s milk lipocalin, which belongs to food allergens [1].
| Animal | Name of protein | Proven cross-reactivity with lipocalins | Can E antibodies to a given protein be determined in tests? |
| cat | Fel d 4 | horse, dog, guinea pig | yes |
| Fel d 7 | dog | yes | |
| dog | Can f 1 | cat | yes |
| Can f 2 | yes | ||
| Can f 4 | yes | ||
| Can f 6 | cat, dog, guinea pig | yes | |
| horse | Equ c 1 | cat, dog, mouse | yes |
| Equ c 2 | no | ||
| mice | Mus m 1 | horse | yes |
| rabbit | Ory c 4 | no | |
| rat | Rat n 1 | mice | no |
| guinea pig | Cav p 6 | dog, cat | no |
| cow | Bos d 2 | yes |
Serum albumins
Another important group of allergenic mammalian proteins in the context of sensitisation to many fur-animals are serum albumins. They are found not only in saliva and epidermis, but also in the blood, milk and meat of animals. These proteins are therefore an important group of inhalant and food allergens [1].
A characteristic feature of albumins is thermolability, which means that proteins from this group lose their allergenicity when exposed to heat treatment. The allergenicity of bovine serum albumin (Bos d 6) is almost completely eliminated when exposed to 180°C for 30 minutes or 220°C for 10 minutes. As a result, people with an allergy to bovine serum albumin can eat beef without the appearance of allergic symptoms [10].
Due to the high degree of similarity between the serum albumins of different animals, cross-reactions within this group are common. An interesting example of cross-reactivity due to this similarity is pork-cat syndrome. It occurs when a person with an inhalant allergy to cat develops allergy symptoms after eating pork. The reason for this reaction is cat serum albumin (Fel d 2), which is very similar to pig serum albumin (Sus s 1) [2]..
| Animal | Name of protein | Proven cross-reactivity with serum albumin | Can E antibodies to a given protein be determined in tests? |
| cat | Fel d 2 | dog, horse, pig, cow, guinea pig, chicken | yes |
| dog | Can f 3 | cat, horse, guinea pig | yes |
| horse | Equ c 3 | cat, dog, pig, guinea pig, chicken | yes |
| guinea pig | Cav p | horse, dog, cat, | no |
| cow | Bos d 6 | cat | yes |
| chicken | Gal d 5 | horse | yes |
Sensitisation to lipocalins or serum albumin does not immediately mean sensitisation and allergic reactions to all animals, although this is a possibility. Therefore, if you are positive for proteins from these families, your allergist will ask you:
- Are you experiencing symptoms?
- What kind of symptoms are they?
- After contact with which furry animals do they occur?
What proteins cause the symptoms of a single animal allergy?
We may also encounter an allergy to just one animal. Thanks to the development of allergology, we have the chance to pinpoint proteins that are more likely to be associated precisely with an allergy to a single furry animal.
Cat secrets
It seems that for the cat such a protein is Fel d 1. It has been shown that up to 95% of cat allergy patients have E antibodies in their blood precisely against this allergen. Fel d 1 is a secretoglobin found in cat saliva and the sebaceous glands of the skin of both sexes, and additionally in the urine of males [5].
If E antibodies to Fel d 1 are found in the blood of a person with allergic symptoms, a diagnosis of primary allergy to the cat can be made. This means that the patient’s symptoms are not the result of cross-reactions related to an allergy to another animal. Elevated IgE levels against Fel d 1 have been linked to the occurrence of asthma in children. It has also been shown that the presence of these antibodies in the blood in childhood without symptoms after contact with a cat is associated with the development of allergy to that animal in teenage years [1].
Interestingly, wild cat species produce proteins very similar to Fel d 1, which can cross-react with secretoglobin. However, under domestic conditions, the likelihood of cross-reactions with this allergen is negligible [6].
Dog speciality
In the case of the dog, the protein responsible for a single animal allergy is Can f 5, or calicrein. This is a unique canine allergen. It is only found in male dogs because it is produced in the prostate, from where it can enter the urine and hair. Neutering dogs has been shown to significantly reduce the amount of Can f 5 they produce. This is important because up to 76% of canine allergy patients may be allergic to this very protein. People who are only allergic to Can f 5 do not experience allergy symptoms as a result of contact with female dogs [1].
A horse with speculations
There is no clearly defined protein that has been linked to allergy to horse only. There is, however, some speculation. Of all the allergens of this animal known so far, laferin, or Equ c 4, has the greatest potential. Sensitisation to this protein is common among people with horse allergy and affects 77-100% of them. Laferin is found primarily in the saliva and sweat of horses. These animals secrete very high amounts of both substances, which is related to thermoregulation [8].
So far, diagnosis of horse allergy is mainly based on the determination of IgE against Equ c 1 (lipocalin) and Equ c 3 (serum albumin). Unfortunately, due to their cross-reactivity, these proteins are not ideal diagnostic markers. The determination of IgE against Equ c 4 may therefore prove useful in the diagnosis of horse allergy. With that said, laferin is not yet a fully understood allergen [1].
As we can see, isolating allergenic proteins indicative of reactions to a specific animal is not easy. Such allergens should be characterised by negligible cross-reactivity, and it can be difficult to meet this condition alone… It must also be remembered that some patients have a co-occurrence of allergy to other groups of proteins, such as lipocalins. In such individuals, allergy symptoms may develop as a result of contact with different groups of animals.
Summary
With component diagnosis, based on the determination of antibodies to specific proteins in the blood, it is possible to find out to which proteins the patient is allergic. It is also possible to establish the likelihood of cross-reactions and to determine which animals the patient will develop symptoms after contact with. Allergy to specific groups of proteins, such as lipocalins, is associated with a more severe course of allergic disease and even asthma. It is important to distinguish between patients at increased risk of asthma, and the determination of antibodies to allergenic proteins of individual animals certainly helps to establish this risk.
