After that read the text aloud, trying to imitate the intonation. All antigens may be formally classified into exogenous and endogenous, depending on their origin

All antigens may be formally classified into exogenous and endogenous, depending on their origin. This distinction is relative. Endogenous and exogenous antigens do not differ in shape, nature, size, smell or taste. The response of the immune system depends very little on whether the antigen is endogenous or exogenous, e.g., exogenous antigens of transfused donor red blood cells are tolerated, whereas the endogenous antigens arising on the recipients' own older erythrocytes incite a cold-blooded murder of these battered veterans by the immune system.

Of much greater significance for the classification of antigens is their division into autoantigens (self) and heteroantigens ( non-self). Endoantigens are not necessarily autoantigens. Autoantigens are biopolymers characteristic of normal unimpaired young cells; although these molecules are superabundant, they normally produce no other response but immunologic tolerance. The markers of senescence expressed on aged cells and pathologic neoantigens induced by viral infection, trauma, and oncogenesis, do not belong to autoantigens. All these antigens are included in a self modified group and treated by the immune system as “aliens”.

On the other hand, heteroantigens are molecules that are not found in normal unimpaired young cells; on the contrary, they appear in sick, older, injured or tumor cells, as well as in alien ones, or invade the organism from outside. These heteroantigens are not normally tolerated but rather produce full-fledged immune responses. No doubt, heteroantigens may be of exogenous as well as endogenous origin (neoantigens and self modified ones). At the same time, there are both endogenous autoantigens (which is only natural), and exogenous (cross-reactive) ones.

According so called “ danger hypothesis ”, all antigens principally can be recognized by immune cells, regardless of their self or alien character. But, alien ones produce more intensive response. It occur because their presence is accompanied by presense of “obligatory pathogenic complexes”, which are viral or prokaryotic biopolymers, absent in eukaryotic organism (in infectious aggression) and also few molecules normally presenting almost exclusively within living cells, like ATP and DNA. The extracellular abundance of such molecules is automatically interpreted as sign of aggression either cell death, which facilitates the longer and more productive cooperation of cells in immune response.

It takes all sorts of antigens to make the immune system work properly, for they are the major object it manipulates.

In fact, an antigen is not only a material molecule, but also an informational unit or signal. As any kind of new information, antigens need presentation for broad recognition and acknowledgement.

According to the classic concept, antigens display epitopes of different types. Conformation determinants represent forms created by tertiary structure of biopolymers. For example, in protein antigens it may be represented by amino acid residues located far from each other in primary polypeptide chains, but put together in spatial three-dimensional globes of fibrils. As a rule, they protrude out of the antigen molecule. Sequential determinants are all but simple and short linear sequences produced by primary structure, for example short peptides of neighboring 9-11 amino acids in protein molecules.

In the course of immune responses to extracellular antigens (during antigen processing after phagocytosis), conformation epitopes are lost, but sequential ones are exposed and presented by antigen-presenting cells on their surface as a rind or panel. In the course of immune responses for intracellular antigens, sequential determinants are created in translation and presented by antigen-presenting cells before a complete set of conformation determinants is acquired in post-translational modifications. Cytophysiological mechanisms of antigen presentation for exocellular antigens (both self and non-self), taken from outside of the antigen-presenting cells, and for endocellular antigens (both self and alien), produced within such cells, are not identical, for example, they involve MHC proteins of different classes and may be displayed in different types of hypersensitivity.

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. All antigens may be informally classified into exogenous and endogenous depending on their origin.

2. Endoantigens are always autoantigens.

3. Unimpaired young cells normally produce immunologic tolerance.

4. The markers of senescence belong to autoantigens.

5. Heteroantigens sometimes invade an organism from without.

6. Heteroantigens can be tolerated.

7. According to the classical concept, antigens display epitopes of different types.

8. Conformation determinants represent forms created by linear structure of biopolymers.

9. Sequential determinants are all but simple and short tertiary sequences.

10. In the course of immune responses to extracellular antigens (during antigen processing after phagocytosis), conformation epitopes are lost, but sequential ones are exposed and presented by antigen-presenting cells on their surface as a rind or panel.

4Correct the false statements from exercise 3.