Antigen drift

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Antigen drift is the slow, continuous and random change in immunity-building surface structures ( antigens ) of viruses .

General

In contrast to the antigen shift , the antigen drift happens randomly and on a smaller scale through copying errors (point mutations) during the replication of viruses . As a result, the viruses can evade the immune system ( immune evasion ) and make systematic medical control more difficult.

This mechanism is known, for example, with the influenza viruses and the lentiviruses , which also include the HI virus .

Requirements for antigen drift

Copy errors in replication are expressed in point mutations , i.e. in the incorporation of wrong bases at random gene locations . If this randomly changed gene location codes an antigen , antigen drift occurs. So one could say that antigen drift is preceded by a genetic drift .

Since viruses, in contrast to the more highly developed cells, have few or no repair mechanisms at all, these errors are not corrected. In addition, the RNA polymerase works relatively inefficiently in comparison to other polymerases, which results in a higher error rate in the strand synthesis and thus makes mutations more likely. The "method of reproduction" of the viruses, which infect foreign cells and produce several thousand copies of themselves there, also proves to be "helpful" for antigen drift. Even with a small chance of a gene change, there is still a good chance that one or more virus molecules will receive a point mutation. While errors of this kind can lead to cell death in highly developed mammalian cells, for example, they even mean a great selection advantage for some viruses (see evolution ).

Antigen drift

The resulting minimal change in certain genes of the viral RNA , which are responsible for coding the respective antigens , results in minimal changes in the structure of the antigens (usually only one amino acid changes ). As a result, the immune system of the infected host cannot recognize the “new” antigens or only with difficulty, and the antibodies previously produced by the immune system can no longer bind the new antigen .

See also: antigenic sin

Significance for the host

The immune system is often able to fight some of the new antigens created by antigen drift with the old antibodies . At some point, however, the antibodies no longer fit, and the immune system has to completely adjust to the antigens.

By the time the immune system has produced the new antibodies , the antigens may have changed again, so that only a small number of viruses can be switched off at any one time. In this way, viruses can repeatedly evade the immune defense , and a persistent (permanent) infection occurs .

Virological importance

Antigen drift does not immediately create new subtypes of viruses. This means that the antigens do not change completely. After all, only a small part of the gene and thus the associated antigen is changed. The classification of the virus subtype based on the envelope proteins therefore remains the same. The emergence of a new subtype requires, among other things, an antigen shift. However, it is conceivable that a new subtype will develop at some point due to ongoing antigen drift.

Importance for combating viruses

Vaccines usually contain a harmless, non-replicable part of the original virus that contains antigens . The immune system makes antibodies in response to the vaccination and, if invaded, can fight the virus immediately before the disease breaks out. Due to the antigen drift, the vaccines are mostly useless, since the real virus has long since changed due to antigen drift before the vaccine is ready for use.

Sometimes, however, the time window is large enough that it is worth using a vaccine, which is then adapted to the new antigens at regular intervals (annual rhythm) . This is the case , for example, with conventional flu vaccinations .

If the time window is too small, however, the only way to combat and prevent the spread is usually in quarantine or - in the case of animals - killing the infected individuals .

Viruses that have antigen drift

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