Basic properties and completeness
To understand the mechanisms of immune defense, it is important to distinguish the basic characteristics of these molecules:
- Antigenicity is the potential ability to act as a specific "stimulus" for immunocompetent cells.
- Immunogenicity is the ability to trigger a full-fledged defense reaction. It depends on molecular weight (strong immunogens are larger than 10 kDa), structure (heteropolymers with L-amino acids and soluble proteins work best), as well as the genetics and physiological background of the macroorganism.
- Foreignness is the structural difference from the body's own tissues.
- Specificity is the uniqueness of the structure that determines the precise direction of the response.
Based on their ability to cause a reaction, substances are divided into two groups:
- Complete antigens. Have a large mass and complex structure. They independently initiate the immune response and bind to its products.
- Haptens (incomplete). Low-molecular-weight substances (e.g., oligopeptides). By themselves, they do not cause a response, but if they bind to a large carrier protein in the body, they acquire the properties of complete antigens.
Structural organization: epitopes
Antigen specificity is determined by epitopes (antigenic determinants), which are small characteristic regions that directly contact the receptors of immune system cells. A single antigen can carry many different epitopes.
- Linear epitopes consist of a continuous chain of 6–12 amino acids. They are recognized by both B lymphocytes and T lymphocytes (for presentation to T helper cells, the epitope length can reach up to 25 residues).
- Conformational epitopes are formed during the spatial folding of a molecule when distant parts of the chain come close together. Such determinants are "seen" only by B lymphocytes and antibodies. Any protein denaturation leads to the destruction of the spatial structure and loss of the original determinants.
Mutation and replacement of just a single amino acid can lead to the formation of a molecule with fundamentally new properties. Vaccine prophylaxis often relies on the addition of adjuvants, which are substances that nonspecifically enhance the immunogenicity of administered antigens.
Microbial antigens and mimicry
A bacterial cell contains a whole set of antigens, knowledge of which allows for the precise identification of the pathogen and the creation of vaccines:
- O-antigen (somatic). Associated with the lipopolysaccharide (LPS) of the outer membrane of gram-negative bacteria. Noted for thermal stability (withstands boiling).
- H-antigen (flagellar). Consists of the flagellin protein. Thermolabile (destroyed at 56–80 °C).
- K-antigen (capsular). Most often has a polysaccharide nature. Interaction with a specific serum causes the visible phenomenon of "capsular swelling." A variant of this—the Vi-antigen—is a marker of high virulence.
Many microbes secrete exotoxins, which are protein antigens. If they are chemically deprived of their toxicity while retaining immunogenicity, toxoids are obtained (the basis for vaccines).
Sometimes microbes form structures similar to normal human tissues. This is called antigenic mimicry. For example, the M-protein of hemolytic streptococcus is similar to myocardial and renal glomerular antigens. During an infection, the immune system begins to cross-attack its own cells, leading to rheumatic fever or post-streptococcal glomerulonephritis.
Erythrocyte antigens (blood groups)
There are over 200 allogeneic antigens on erythrocyte membranes. The clinically most significant are the AB0 and Rh (rhesus factor) systems.
The AB0 system is based on the basic H antigen, from which antigens A and B are synthesized. Their combination determines the blood group (for example, group I lacks antigens A and B, while group IV contains both). If incompatible blood is transfused into a person, antibodies attack the foreign erythrocytes. Agglutination and intravascular hemolysis develop, leading to severe hemolytic shock.
Rhesus conflict often occurs during pregnancy if the mother is rhesus-negative and the fetus is rhesus-positive. The mother's immune system produces antibodies that destroy the child's erythrocytes, causing neonatal jaundice syndrome.
Briefly: answers to topical questions
What is the role of the major histocompatibility complex (MHC) in antigen presentation to T lymphocytes?
The major histocompatibility complex (MHC) is involved in presenting antigenic peptides to T lymphocytes to initiate a specific immune response. Antigen-presenting cells capture the pathogen, break it down into peptide fragments, and display the antigen fragment on their surface in a complex with an MHC molecule.
- MHC class I and class II encode molecules that perform antigen presentation to T lymphocytes.
- MHC class II presents antigen fragments to CD4+ T lymphocytes (T helper cells).
The interaction of the T-cell receptor with the "antigen + MHC" complex provides the specific signal required for the full activation and proliferation of T lymphocytes.
To easily remember the localization of bacterial antigens: O — base (somatic, in the wall itself), H — thread (flagellar, outside), K — capsule (surface layer).
Class and exam questions
What is antigenic mimicry?
This is a mechanism by which a microorganism synthesizes antigens structurally similar to host tissues. As a result, the immune system is activated against the infection, but cross-attacks its own organs as well, causing autoimmune diseases.
How does a hapten differ from a complete antigen?
A hapten is a low-molecular-weight compound. It is capable of binding to preformed antibodies, but cannot independently initiate an immune response until it forms a complex with a large carrier protein.
What is a toxoid?
This is a bacterial exotoxin that has been deprived of its toxicity to the macroorganism through special processing while fully retaining its immunogenicity. It is used as a molecular vaccine.
What else to study on this topic
- Mechanisms of formation and breakdown of immunological tolerance (autoantigens and autoimmune diseases).
- Superantigens: nonspecific polyclonal activation of lymphocytes and apoptosis.
- Classification of antigens by genetic relationship: xenogeneic, allogeneic, isogeneic, syngeneic.
- Differences in immune activation by T-dependent and T-independent antigens.
Study the full topic in the «Microbiology» course
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