Since the antigenicity of the antigen was found in vitro, the serological method was used in vitro experiments to prove that the natural antigen specifically binds to its corresponding antibody. This is an important immunological phenomenon, and this characteristic is called the antigenicity of the antigen. In the early days, it was difficult to study the chemical nature of antigenicity because there was no established method for analyzing protein antigens. Austrian immunochemist Landsteiner created artificially combined antigens in the 1920s, and applied serological methods to systematically study the chemical nature of antigenicity, providing a lot of knowledge for the interaction between antigens and antibodies, and the chemical properties of natural antigens. Research has laid the foundation.
1. Artificially bound antigen
Landsteiner pioneered the use of low-molecular compounds or chemical groups with known chemical structures to couple with immunogenic proteins to prepare azoamine proteins. Because the low-molecular compound itself is not immunogenic, animals that are immunized alone cannot produce antibodies to them. If coupled to a protein and then immunized, the animals can produce antibodies to them and can bind to the antibodies alone. Therefore, this non-immunogenic low molecular compound is called a hapten (hapten, H), and the immunogenic protein is called a carrier (cauuier, C). Since then, the concept of hapten and carrier (HC) has been established. Therefore, hapten can be considered as a chemical group with a known chemical structure added to the immunogen molecule. In a certain sense, it is synonymous with the natural antigenic determinant of the unknown chemical structure on the antigen molecule (Figure 10-1).
Immunogenic proteins commonly used as carriers are bovine serum albumin (BSA), bovine serum gamma globulin (BGG), ovalbumin (OA), chicken gamma globulin (CGG), keyhole limpet hemocyanin (KLH) and Synthetic polylysine (PLL), etc. Commonly used haptens with known structure include dinitrophenol (DNP) and trinitrophenol and other low molecular weight compounds.
Landsteiner has organic substances with different acid groups as haptens. They were coupled with the same carrier to prepare several hapten carrier conjugates with different acid groups, and then immunized the animals. The obtained antibody was used to perform in vitro complement binding experiments with known haptens. The results proved that haptens with different acid groups could only bind to their corresponding antibodies. In the same way, he uses three isomers of aminobenzoic acid (ortho, meta, para) to hapten carrier conjugates coupled to the same carrier protein, and can only bind to their corresponding antibodies (Table 10-1 ,2). The above experiments prove that the specific binding of the antigen to the antibody is related to the chemical group of the special structure on the surface of the antigen molecule. The part of the surface of the antigen molecule that can bind to its antibody is called the epitope, that is, the antigenicity of the antigen is determined by the nature, number and spatial configuration of the antigenic determinant. Since then, the concept of antigenic determinant has been established. It provides a theoretical basis for future research on the antigenicity of natural proteins.
In addition to the specific reaction between the antigen and its corresponding antibody, it can also react with other related antibodies. This phenomenon is called cross-reaction. This is caused by having the same antigenic determinant in two different antigen molecules, called a common cluster or common antigen. It may also be because the two different determinants have similar structures, but not the same, and can also cause cross-reactions . Among the species that are close in occurrence, the antibodies produced by their antigens are prone to cross-react with each other. Such as the cross-reaction between bovine serum albumin (BSA) and albumin of other species (Table 10-3).
2. Carrier determinants and hapten determinants
The use of hapten alone cannot induce the body to produce antibodies. Only by combining the hapten with the carrier protein can the body be induced to produce both anti-carrier protein antibodies and hapten antibodies, which has been proved by experiments. This raises questions such as why hapten alone cannot produce antibodies and what role the carrier plays in the production of antibodies.
In order to answer the above questions, some scholars used bovine serum albumin-biphenylbenzene (BSA-DNP) and ovalbumin-dinitrobenzene (OA-DNP) as carriers to bind haptens. In the role of production, they first applied BSA-DNP to immunize the mice for the first time, and then used BSA-DNP and OA-DNP to immunize again, and observed the effect of the carrier against DNP antibodies. They found that only when the first time and again During immunization, haptens need to be on the same carrier to produce hapten antibodies, which is called the carrier effect (Table 10-4). It proves that the carrier does not simply carry the hapten, but has carrier specificity. Therefore, it is proposed that a complete antigen molecule must have a carrier determinant and a hapten determinant.
Later, Mitchison et al. Used carrier effect adoptive transfer experiments in the 1970s, proving that there are cells specific for carriers and cells specific for haptens during antibody formation, which are called carrier-responsive cells and hapten-reactive cells, respectively. And further proved that T cells are carrier-responsive cells, which play an auxiliary role in antibody production. B cells are hapten-responsive cells and antibody-producing cells, which has clarified the cytological basis of the carrier effect.
The carrier-hapten concept is extremely important. It can explain why low molecular weight compounds combine with carrier protein molecules in the body to induce hypersensitivity reactions to drug allergies. Such as aniline dyes, sedatives, carbendazim, antipyretic agents aspirin, aminopyrine, and various antibiotic decomposition products are the causes of drug allergies.
3. Antigenicity of Natural Protein
The antigenicity of antigens The vast majority of immunogenic substances are macromolecular proteins or cellular components, but only a limited part of them can bind to their corresponding antibodies. Prove that an antigen molecule must have T cell determinants and B cell determinants). Due to the complex structure and spatial configuration of natural macromolecular proteins, it is very difficult to study the chemical composition, location and structure of its single epitope. However, in recent years, due to the development of immunochemical technology, its spatial structure has been able to be studied. Due to the development of monoclonal antibody technology, it has been possible to isolate and identify single antigenic determinants. Therefore, great progress has been made in the study of protein antigenicity.
(1) B cell determinant (epitope)
The B cell determinants of antigen molecules are different in size, and their maximum surface area is about 50-70 mm, which is composed of 4-6 amino acid residues or sugar groups. A 100 amino acid residue polypeptide can have 14 to 20 non-overlapping determinants, which are composed of linearly arranged amino acids adjacent to each other, so they are called linear or continuous determinants. And globulin is a folded peptide chain with three-dimensional space, so most of its determinants are covered inside, which can be called concealed determinants. The determinants that exist only on the surface can be recognized by immune cells, or those that bind to antibodies are called functional determinants (Figure 10-2). The amino acids that make up this determinant are the folded peptide chains that make the amino acids in the same position adjacent to each other into a determinant with a certain spatial configuration, so they are called conformational determinants or discontinuous determinants. It has been proved that the antigenicity of the antigen molecule is determined by the amino acid sequence, spatial configuration and movement of its determinant fragments.
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