Complete and Incomplete Antigens
In immunology, substances capable of triggering immune responses are divided into two categories based on their independence.
Complete antigens initiate an immune response independently. These are typically high-molecular-weight compounds (foreign proteins or polysaccharides). They exist in two forms:
- Particulate: Associated with 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, although their recognition mechanisms may differ.
Haptens (incomplete antigens) are a group of substances (foreign peptides, DNA, lipids, low-molecular-weight organics) that are harmless by themselves and do not provoke a response. A hapten becomes immunogenic only when bound to a high-molecular-weight carrier (a protein or polysaccharide) that was not originally an antigen to the host organism.
Antigenic Determinants (Epitopes)
The immune system does not recognize an entire molecule, but rather its small fragments — antigenic determinants (AD), or epitopes. These specific regions are recognized by B- and T-cell 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, specific oligosaccharides.
As a rule, a single protein contains multiple different antigenic determinants. Because microorganisms consist of a vast number of proteins, infection results in the presentation of numerous distinct epitopes. This leads to polyclonal activation — the simultaneous activation of multiple different B- and T-lymphocyte clones.
Antigen Recognition by Lymphocytes
Differences in antigen recognition mechanisms are 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 free in solution or anchored to a cell surface.
T lymphocytes (cytotoxic T cells and helper T cells) fundamentally cannot see native antigens. Their activation requires a dual signal: the peptide itself and a major histocompatibility complex (MHC) molecule, which acts as a cellular platform to display the epitope.
Intracellular Processing Algorithm
For T lymphocytes to detect an antigen, the cell must process it intracellularly. This process consists of four stages:
- Cleavage: An endogenous or exogenous antigen is degraded inside the cell into fragments.
- Binding: A specific peptide binds to an MHC molecule.
- Transport: The formed complex is translocated to the cell membrane.
- 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 T-cell type and the MHC molecule:
- For cytotoxic T cells (CD8+): A short peptide (8–11 amino acids) is displayed in a complex with MHC Class I. These molecules are found on the surface of almost all nucleated cells in the body, as cytotoxic T cells survey all tissues.
- For helper T cells (CD4+): A larger peptide (15–20 amino acids) is presented on an MHC Class II platform. These complexes are localized exclusively on the membranes of professional antigen-presenting cells (APCs).