What are antigenic determinants?

People who have studied biology or medicine must be familiar with the term antigenic determinant cluster. It is mainly composed of proteins and exists on the surface of antigen molecules or other structures in the human body. It is divided into two categories according to whether it is an antigen or not. One is the antigenic determinant cluster and the other is the immunogenic determinant cluster. It promotes the development of the human immune system. The following editor will introduce the antigenic determinant cluster in detail.

It can be composed of a continuous sequence (protein primary structure) or a discontinuous protein three-dimensional structure. It is a special chemical group that determines antigenicity, also known as an antigenic epitope. Most antigenic determinants exist on the surface of antigenic substances, while some exist inside the antigenic substances and are only exposed after being treated with enzymes or other methods. A natural antigenic substance can have multiple and multiple determinants. The larger the antigen molecule, the greater the number of determinant clusters.

Classification

Most antigenic determinants exist on the surface of antigenic substances, while some exist inside the antigenic substances and are only exposed after being treated with enzymes or other methods. A natural antigenic substance can have multiple and multiple determinants. The larger the antigen molecule, the greater the number of determinant clusters. Among various antigenic determinants, the immunogenic determinants are the most likely to induce an immune response. Determinants can be further divided into two categories:

① Antigenic determinant cluster. It acts on B cells and can bind to the Fab fragment of the counterpart.

②Immunogenicity determinant cluster. Finally, it acts on T cells and is related to cellular immunity.

The B cell determinant clusters of antigen molecules vary in size, with a maximum surface area of ​​about 50 to 70 mm and composed of approximately 4 to 6 amino acid residues or sugar groups. A polypeptide of 100 amino acid residues may have 14 to 20 non-overlapping determinant clusters, which are composed of amino acids arranged adjacent to each other in a linear manner, so they are called linear or continuous determinant clusters. Globulin is a folded peptide chain in three-dimensional space, so most of its determinants are hidden inside, which can be called hidden determinants. The determinants that only exist on its surface and can be recognized by immune cells or bind to antibodies are called functional determinants. The amino acids that make up this determinant cluster are formed by the folded peptide chain making the amino acids in the same position adjacent to each other to form a determinant cluster with a certain spatial configuration, so it is called a conformational determinant cluster or a discontinuous determinant cluster. It has been proven that the antigenicity of an antigen molecule is determined by its amino acid sequence, spatial configuration, and the movement of its determinant cluster fragments.

T cell determinant clusters are immunogenic polypeptide fragments and are continuous determinant clusters. However, it does not exist on the surface of natural protein molecules and must be processed by antigen-presenting cells into small peptide molecules and then combined with their own MHC molecules before they can be recognized by T cells. B cell determinant clusters can exist on the surface of natural antigen molecules and can be directly recognized by B cells without any processing.

It is very difficult to directly prove the existence of T cell determinant clusters and B cell determinant clusters in natural macromolecular protein antigens. However, studies using small molecule immunogens of known structure have demonstrated the existence of two determinant clusters. The pancreatic hormone glucagon is composed of 29 amino acids and is immunogenic. The anti-hormone antibodies produced by it recognize its amino terminus. The killer T cells produced by it can recognize its carboxyl terminus, which is equivalent to the carrier part of the hormone molecule. This proves that the amino terminus of the glucagon molecule is a B cell determinant cluster, while the hydroxyl terminus is a T cell determinant cluster.

The location of these two determinant clusters of antigenic substances determines the specificity of humoral immunity and cellular immunity. The size of a protein antigenic determinant generally does not exceed 6 to 8 amino acid residues; a carbohydrate antigenic determinant contains approximately 6 units of hexose (six-carbon sugar); and each antigenic determinant of a nucleic acid hapten contains approximately 6 to 8 nucleotides. The specificity of an antigenic determinant depends not only on its amino acid composition, number and arrangement order, but also on the local configuration of the molecule and the influence of the rest of the molecule on this local configuration.