Antigens

Complete and incomplete antigens

In immunology, substances capable of triggering defense reactions are divided into two categories depending on their independence.

Complete antigens initiate an immune response on their own. These are typically high-molecular-weight compounds (foreign proteins or polysaccharides). They exist in two forms:

• Corpuscular: bound to large particles, present on the membranes of viruses, bacteria, eukaryotic, or tumor cells.
• Soluble: circulate in body fluids in molecular form.

Both forms successfully initiate the immune process, but the mechanisms of their recognition may differ.

Haptens (incomplete antigens) are a group of substances (foreign peptides, DNA, lipids, low-molecular-weight organics) that are safe on their own and do not cause a response. A hapten becomes immunogenic only when bound to a macromolecular carrier (a protein or polysaccharide) that was not initially an antigen for the given organism.

Antigenic determinants (epitopes)

The immune system recognizes not the entire molecule, but only its small fragments — antigenic determinants (AD), or epitopes. It is these regions that are recognized by B- and T-lymphocyte receptors and determine the strict specificity of the reaction.

The chemical nature of epitopes depends on the molecule:

• In proteins, this role is performed by peptide fragments.
• In polysaccharides, by specific oligosaccharides.

As a rule, a single protein contains many different antigenic determinants. Since microorganisms consist of a huge number of proteins, infection presents a multitude of different ADs. This leads to response polyclonality — the simultaneous activation of many different clones of B- and T-lymphocytes.

Antigen recognition by lymphocytes

The difference in antigen recognition mechanisms is key to understanding the entire immune response.

B-lymphocytes are capable of recognizing antigenic determinants within native (intact, unaltered) macromolecules. It does not matter to them whether the antigen is in free form in solution or anchored to the cell surface.

T-lymphocytes (T-killers and T-helpers) fundamentally cannot recognize native antigen. Their activation requires a dual signal: the peptide itself and a major histocompatibility complex (MHC or MHC) molecule, which acts as a cellular platform to present the epitope.

Algorithm of intracellular processing

For T-lymphocytes to detect an antigen, the cell must undergo intracellular processing of it. This process consists of four stages:

1. Cleavage: an external or internal antigen is cleaved inside the cell into fragments.
2. Binding: a specific peptide binds to an MHC molecule.
3. Transport: the formed complex is delivered to the cell membrane.
4. Screening: T-lymphocytes inspect the cell surface with their receptors (TCR) until they find a matching complex.

There is a strict dependence (MHC restriction) between the type of T-lymphocyte and the MHC molecule:

• For cytotoxic T cells: a short peptide (8–11 amino acids) is displayed on the surface in a complex with MHC class I. These molecules are present on the surface of almost any cell in the body, as cytotoxic T cells inspect all tissues.
• For T helper cells: a larger peptide (15–20 amino acids) is presented on the MHC class II platform. Such complexes are localized exclusively on the membranes of professional antigen-presenting cells (APCs).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Functions and types of professional antigen-presenting cells (APCs)
• Differences in response mechanisms to soluble and particulate antigens
• Molecular structure of T-lymphocyte receptors (TCR)
• Biochemistry of major histocompatibility complex (MHC) class I and II
• Mechanisms of polyclonal immune response formation

Other topics in the section "Hematopoietic and lymphoid organs"

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