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Diagnostic Agents

Q: What classic skin tests (in vivo) are used for diagnosing specific bacterial infections?

For diagnosing bacterial infections, in vivo skin-allergy tests based on delayed-type hypersensitivity to specific allergens are employed. The following diagnostic agents are used: Tuberculin (M. tuberculosis) — diagnosis of tuberculosis (Mantouxtest).Brucellin — diagnosis of brucellosis (Burnet test).Tularin — diagnosis of tularemia.Lepromin — diagnosis of leprosy.Mallein — diagnosis of glanders.Anthraxin — diagnosis of anthrax.Pestin — diagnosis of plague. These agents are administered intradermally or percutaneously, and results are evaluated after 48–72 hours based on the presence and size of the induration.

For medical students2 min readUpdated 2026-10-10

Diagnostic agents are specialized biological and chemical products designed for the immunodiagnosis of infectious and non-infectious pathologies, as well as for evaluating a patient's overall immune status. Their mechanism of action is based on the precise detection of specific humoral and cellular immune reactions, which can occur either in vitro (in a laboratory test tube) or in vivo (directly within the organism).

Primary targetCellular and humoral immune reactions

SpecificityExceed biochemical methods in specificity

DetectionRegistration of physical and chemical reaction effects

Spectrum of infectionsFrom anthrax and diphtheria to HIV and hepatitis

Main Groups of Diagnostic Methods

The spectrum of applications for diagnostic agents encompasses numerous analytical approaches that allow for a detailed assessment of the body's physiological state. The technical requirements for these methods range from basic manual assays (performed on a microscope slide or in a test tube) to advanced computerized systems.

Key directions include:

Antigen-antibody reactions. Reagents include both natural antigens and antibodies as well as modern high-tech equivalents: monoclonal antibodies, synthetic peptides, and recombinant proteins.
Molecular genetic testing. Includes genetic titration based on amplification (PCR) and molecular hybridization.
Cellular assays. Aimed at quantifying immunocompetent cells (antigen-presenting cells, T and B lymphocytes) and evaluating their functional activity.
Evaluation of innate immunity factors. Analysis of the complement system, lysozyme levels, and interferons.
Measurement of soluble mediators. Quantifying concentrations of cytokines and other molecules that regulate the immune response.
Skin tests. A classic example of diagnostic reactions performed in vivo.

Medical Applications

Modern diagnostic agents possess high sensitivity and specificity, making them more effective than standard endoscopic, radiological, or biochemical methods in many clinical contexts. They are actively used in several fields:

Infectious diseases. Indispensable for identifying viral and bacterial agents. They are used to diagnose typhoid fever, plague, tularemia, anthrax, diphtheria, HIV infection, and viral hepatitis.
Allergology. Detection of specific reactions to food, occupational, and other types of allergens.
Oncology. Localization of malignant neoplasms, including cancers of the lung, liver, and colorectum.
Transplantation. Determination of tissue compatibility, alongside early diagnosis of graft tolerance or rejection reactions.
Obstetrics. Analysis of complex immunological interactions in the maternal-fetal system.
Immune status monitoring. Identification of immunodeficiency states and assessment of how adverse social or environmental factors impact the body's defense systems.

Biosensor Test Systems

A distinct category consists of biosensor test systems. Their operation is based on registering chemical or physical changes that occur precisely at the moment a specific immune reaction takes place. These effects are recorded by ultrasensitive detectors.

Physical parameters: Detectors capture agglutination, opalescence, various types of radiation, or changes in thermal background. For instance, heat release during antigen-antibody binding can be recorded.
Chemical parameters: Systems record the formation of new compounds or reaction products. A typical example is the detection of carbon dioxide ($CO_2$) released during the interaction between an enzyme and its substrate when using enzyme labels.

The diagnostic utility of such systems is continually increasing due to the implementation of purified antigens and monoclonal antibodies.

Frequently asked questions

Which immunoglobulin classes are measured in blood serum to confirm the acute phase of an infectious disease?

To confirm the acute phase of an infectious disease, class M immunoglobulins and early antibodies of other classes are determined in blood serum. Identifying these markers helps clarify the stage of the process and differentiate an active infection from post-vaccination immunity.

Immunoglobulin M (IgM) — synthesized first in response to infection and serves as the primary marker of the acute phase and primary infection.
Immunoglobulin G (IgG) — during the acute phase, these may appear as low-avidity antibodies directed against early viral proteins.

What classic skin tests (in vivo) are used for diagnosing specific bacterial infections?

For diagnosing bacterial infections, in vivo skin-allergy tests based on delayed-type hypersensitivity to specific allergens are employed. The following diagnostic agents are used:

Tuberculin (M. tuberculosis) — diagnosis of tuberculosis (Mantouxtest).
Brucellin — diagnosis of brucellosis (Burnet test).
Tularin — diagnosis of tularemia.
Lepromin — diagnosis of leprosy.
Mallein — diagnosis of glanders.
Anthraxin — diagnosis of anthrax.
Pestin — diagnosis of plague.

These agents are administered intradermally or percutaneously, and results are evaluated after 48–72 hours based on the presence and size of the induration.

What problems do diagnostic agents solve in transplantation, and what methods determine tissue compatibility?

In transplantation, diagnostic agents are used to assess organ and tissue compatibility and to diagnose rejection and tolerance reactions. To select a donor-recipient pair, MHC typing is performed using the following methods:

Serological method — microlymphocytotoxicity assay utilizing B lymphocytes, monospecific antisera, and complement.
Molecular genetic method — DNA typing based on the polymerase chain reaction (PCR).

Modern PCR typing allows for the direct detection of MHC genes rather than their protein products.

What is the main advantage of immunological diagnostic methods?

They offer significantly higher specificity and sensitivity compared to biochemical, endoscopic, and radiological examinations.

What modern reagents are used in antigen-antibody reactions?

In addition to natural components, modern practice widely utilizes monoclonal antibodies, recombinant proteins, and synthetic peptides.

How do biosensor detectors work?

They register physical (heat, radiation, agglutination) or chemical (gas production, novel products) effects generated during specific immune interactions.

Go deeper

Molecular genetic diagnostics: principles of PCR and hybridization
Evaluation of the functional status of T and B lymphocytes
The role of cytokines as soluble mediators of the immune response
Immunological aspects of transplantation and rejection reactions
Innate immunity factors: complement system and lysozyme