After that read the text aloud, trying to imitate the intonation. The living organism persists in the unfriendly environment in spite of mechanical, physical, chemical and biological pathogenic factors (pathogens) tending to

The living organism persists in the unfriendly environment in spite of mechanical, physical, chemical and biological pathogenic factors (pathogens) tending to harm it from outside and a lot of unfavorable tendencies injuring it from inside. The only way for an organism to survive and flourish is to have all its systems permanently strained in order to achieve a balance between all kinds of damaging influences and the efforts of the organism to compensate, thus maintaining its homeostasis.

The role of the immune system in achieving this goal is unrivalled, because the immune system sustains the homeostasis on the sub-cellular level, being uniquely equipped with a mechanism capable of producing a host of special tools, each of them specifically complementary to one out of a billion various biopolymer molecular types. These molecules are called antigens and the specific tools of the immune system are referred to as antigen-specific lymphocyte receptors.

A substance is antigenic provided it has a three-dimensional structure capable of key-lock complementary principle interactions. To put it in other words, it must be able to recognize and to be recognized. Paul Ehrlich[23] formulated this principle in a proverbial form: “Corpora non agunt nisi fixata.” – “No bodies can act unless they bind (recognize)”. Antigenicity requires some degree of structural complexity and spatial rigidity.

Even high molecular weight substances can be non-antigenic if they have a flat structure (cholesterol). Gelatin is a high molecular weight protein devoid of tertiary structure, and, consequently, it is non-antigenic or is regarded as very weak antigen (unable to form “key-hole”, according the example given by W. Boyd)[24].

Thus, antigens are biopolymeric molecules capable of inciting immune response in the organism by specifically binding to the lymphocytic antigenic receptors.

Antigens are a wide variety of natural and biosynthetic moleculesranging in size between 1 and 10 kD (proteins, polysaccharides, complex esters and heterocyclic substances, nucleic acids, polynucleotides and their complexes, including lipid-containing ones).

The definition of an antigen has changed drastically during the last 60 years. Initially, it was regarded simply as a synonym of a “pathogen”; later, immunologists re-interpreted this term as a direct complementary cast of an antibody, and, finally, it came to be defined as mentioned above. Hence, antigens are polymeric molecules that can trigger specific immune responses when introduced into the organism. Interestingly, only polar molecules are highly immunogenic.

At the same time, the wide-spread popular interpretation of the word “antigen” as an “alien protein” must be rejected from the very beginning of the present discussion because of being totally obsolete. Moreover, many low molecular weight substances (for instance, iodine) may generate unique three-dimensional complexes with biopolymeric carriers, which provoke immune responses. Such low molecular weight substances are called haptens.

The immunochemical treatment of an antigen is possible only if it has covalent chemical bonds. From this standpoint, cells, bacteria and multimolecular complexes possess many antigenic specificities. The minimal structural unit of an antigen, recognizable for antibodies and lymphocyte antigenic receptors, is known as an epitope (immunodominant region).

As a rule, epitopes are located in the most mobile and configurable parts of quite rigid antigen structures. The theory of clonal selection insists, that each clone of lymphocytes is genetically programmed to recognize only one epitope by means of an antigen-specific receptor on its surface, so that these cells display fine antigen specificity. It is believed that billions of different epitopes may exist, consequently, the equivalent number of lymphoid cell clones with their unique receptors is required for the recognition of this multitude.

Any immune response requires a certain densitiy of epitopes: it disappears if their compactness is either too high or too low. Single epitopes are non-immunogenic, since for an immune response to occur the recognizing molecules must be cross-linked. Due to this overwhelming majority of immune responses the organism needs cooperation of several specialized cells (B-lymphocytes, T-lymphocytes, antigen-presenting cells).

Some antigens include panels of numerous identical epitopes, among them: bacterial lipopolysaccharides, polymeric protein of Flagellates–flagellin, dextran, levan and other bacterial capsule polysaccharides, ficoll and polypeptides composed of D-aminoacids. They are hardly digestible for antigen-presenting cells, but able to cross-link the recognizing lymphocyte receptors directly. This group of antigens provokes polyclonal B-cell activation and immunoglobulin production without T-cell cooperation. Many individual antigens have several or even numerous different epitopes and provoke simultaneous immune response of various cellular clones (both antibody secretion and clonal expansion of cellular effectors).

3 Do the following statements agree with the information given in the text?

Write

TRUE if the statement agrees with the information

FALSE if the statement contradicts the information

NOT GIVEN if there is no such information

1. Any living organism has to sustain a variety of pathogenic factors to survive.

2. Pathogens tend to destroy an organism from inside.

3. Homeostasis is maintained by the immune system.

4. A substance is antigenic as it has the ability to recognize and to be recognized within immune system.

5. A substance is antigenic provided it has a three-dimensional structure.

6. High molecular weight substances are non-antigenic if they have a flat structure.

7. In accordance with classical views gelatin is either non-antigenic or very weak antigen.

8. Antigens stimulate immune response via binding to the lymphocytic antigen-specific receptors.

9. Antigens are a limited variety of natural and biosynthetic molecules.

10. Haptens are high molecular weight substances.

11. The immunochemical treatment of an antigen is possible only if it has divalent chemical bonds.

12. An epitope is the minimal structural antigenic unit.

13. It is believed that billions of different cell clones are required for the recognition of billions of different epitopes.

14. Each clone of lymphocytes is genetically programmed to recognize several epitopes.

15. B-lymphocytes and T-lymphocytes can cooperate with single epitopes.

4Correct the false statements from exercise 3.