IMMUNOLOGY - Roitt I. - Mir 2000

Chapter 1. General Overview of the Immune System

ANTIGENS

Originally, the term antigen (derived from antibody generator) was used to describe any molecule that induces B Cells to produce specific Antibodies. However, the term now has a broader meaning, referring to any molecule that can be specifically recognized by elements of the adaptive immune system, namely B cells, T cells, or both.

Antibody molecules do not bind to the entire surface of an infectious agent. True to their Specificity, each antibody interacts with one of the many types of antigenic molecules On the surface of microbes. Several antibodies with different specificities can be synthesized against a single pathogen, binding to different Antigens on its surface. Antibodies interact with a specific region of the antigen molecule known as an epitope. A single antigen may possess several different or repeating epitopes (Fig. 1.11). Antibodies are specific precisely to these epitopes rather than to the antigen molecule as a whole.

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Fig. 1.11. Molecules that trigger antibody production are called antigens. Each antigen molecule features a set of antigenic determinants called epitopes. The epitopes of one antigen (Ag1) typically differ from those of another (Ag2). Some antigens (Ag3) contain repeating epitopes. The stereochemical configuration of these epitopes is recognized by antibodies and T-Cell receptors, which are the effectors of adaptive Immunity. Each antibody molecule recognizes not the whole antigen, but a single epitope. Even the structurally simplest microorganisms possess a multitude of distinct protein, lipid, or carbohydrate antigens.

The mechanisms that generate a repertoire of antibodies diverse enough to bind all the various antigens an Organism may encounter throughout its lifetime are described in Chapter 8.

Antigen recognition is the foundation of adaptive immunity

In addition to antibodies and B cells, T cells also participate in antigen recognition. However, T cells recognize antigens in the form of small polypeptide fragments that are initially localized intracellularly and subsequently displayed on The surface of other host cells. For example, virus-infected cells begin to express small fragments of viral Proteins on their cell surface, making them easily recognizable by cytotoxic T cells. The presentation of antigen fragments on The Cell surface is mediated by a specialized group of so-called MHC molecules, encoded by the GENES OF THE Major Histocompatibility Complex (MHC). T cells recognize peptide fragments of the antigen bound to these MHC molecules via their antigen-specific receptors (TCRs) (Fig. 1.12).

Fig. 1.12. T cells recognize antigens that are initially localized intracellularly and subsequently appear on the surface of other cells, such as viral Peptides from infected cells. Recognition occurs through specific binding to antigenic peptides presented on the cell surface by MHC molecules—products of the major histocompatibility complex (MHC) genes. T cells recognize the unique complex of the antigenic peptide and the MHC molecule via their antigen-specific receptors (TCRs). Unlike B cells, which recognize a specific region of an intact antigen molecule, T cells recognize an epitope formed by The amino acid residues of both the antigenic peptide and the MHC molecule.

It is important to remember that an antigen is the initiator and driving force of all adaptive immune responses. The Immune System evolved to recognize and destroy foreign antigens, as well as to eliminate their source—Bacteria, virus-infected cells, and the like. Once the antigen is eliminated, the Immune Response subsides.