Immunological basis and status of the method
In microbiological and infectious disease practice, the allergological method is not universal. It has an auxiliary status and is used only for a specific, strictly limited range of infections.
The foundation of this type of diagnostics is the ability of the immune system to form specific memory. Upon contact with a pathogen or after administration of a vaccine strain, the body becomes sensitized. If such a patient is administered a specific antigen (allergen), a delayed-type hypersensitivity (DTH) reaction develops, which is designated as a type IV reaction in the Gell and Coombs classification. The main way to visualize this reaction is by performing skin allergy tests.
Technique and interpretation of results
For diagnostics, the patient is administered a standardized diagnostic preparation containing antigens of the target microorganism.
There are two main methods of allergen administration:
- Intradermal — most commonly used. The preparation is injected into the middle third of the flexor surface of the forearm.
- Percutaneous (scarification test) — used less frequently. In this case, the allergen is applied to the skin, after which superficial scratches (scarifications) are made through the drop of the preparation.
Since the basis is delayed-type hypersensitivity, an immediate reaction does not occur. The results are evaluated strictly in a delayed manner — after 48–72 hours. The main diagnostic criterion confirming the body's sensitization is the formation of an inflammatory infiltrate (papule) at the injection site. The physician assesses the very fact of the papule's presence and measures its size.
Diagnostic preparations
For each infectious disease, a specific preparation is used, the name of which is most often derived from the name of the pathogen.
| Drug | Infectious disease | Note (eponym test) |
|---|---|---|
| Tuberculin | Tuberculosis | Mantoux test |
| Brucellin | Brucellosis | Burnet test |
| Tularin | Tularemia | - |
| Toxoplasmin | Toxoplasmosis | - |
| Mallein | Glanders | - |
| Anthraxin | Anthrax | - |
| Pestin | Plague | - |
| Ornithin | Ornithosis | - |
Allergy to opportunistic flora and in vitro tests
Sensitization of the body is possible not only to strict pathogens, but also to representatives of its own normal or opportunistic microflora. The development of DTH to such microbes is of immense clinical significance: this allergy often becomes the leading mechanism in the pathogenesis of various chronic diseases. Various organ systems can be affected: the gastrointestinal tract, respiratory tract, kidneys, skin, and joints.
Allergens maintaining such chronic inflammation can include:
- Staphylococci and streptococci;
- Pneumococci;
- Escherichia (E. coli);
- Proteus;
- Pseudomonas aeruginosa.
In addition to classical skin tests on the patient (in vivo), there are laboratory variations of the allergological method performed in vitro (in vitro). These include the neutrophil damage test and the lymphocyte blast transformation test with the addition of corresponding allergens.
To easily remember the names of the preparations, pay attention to their suffix "-in". The root of the word always indicates the disease or pathogen (Brucellosis — Brucellin, Plague (Pestis) — Pestin). Exception requiring memorization: Mallein is used for the diagnosis of glanders.
Class and exam questions
Why is the allergological method called auxiliary?
Because it only detects the fact of the body's sensitization (past or present exposure to an infection or vaccine) rather than the pathogen itself. Furthermore, it is applicable only to a limited list of infections.
How soon should the skin reaction be checked?
Skin allergy tests are always evaluated after 48–72 hours. This time is necessary for the development of a cellular immune response (DTH).
How to determine that the reaction is positive?
The main criterion for a positive reaction is the formation of a papule (dense infiltrate) at the site of administration. Both its presence and size are taken into account.
What else to study on this topic
- Pathogenesis of the delayed-type hypersensitivity reaction (type IV according to Gell and Coombs)
- Principles of performing the blast transformation test with allergens
- Mechanism of the in vitro neutrophil damage test
- Role of opportunistic flora in the development of autoimmune and chronic pathologies
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