Antigenic drift

Antigenic drift is the slow, continuous and random change of immunity- forming surface structures ( antigens ) of viruses.

General

In contrast to the antigenic drift antigenic shift happens randomly and on a smaller scale by copying errors ( mutations ) during the propagation ( replication ) of viruses. The viruses can evade the immune system and thereby complicate the systematic medical control.

This mechanism is known, for example on influenza viruses and lentiviruses, including HIV belongs.

Conditions for antigenic drift

Copying errors during replication are expressed in point mutations, ie in the installation of wrong bases at random loci from. This locus encodes an antigen randomly changed, it comes to antigenic drift. So you could say that the antigenic drift is preceded by a genetic drift.

Since viruses, in contrast to the more developed cells have few or no repair mechanisms, these errors are not corrected. In addition, the RNA - polymerase compared to other polymerases operates relatively inefficiently, resulting in a higher error rate results in strand synthesis and mutations are hereby probable. While errors of this type, for example, in a highly developed mammalian cell can lead to cell death, they mean for some viruses even a large selection advantage (see Evolution).

Antigenic drift

Formed by the minimal change of certain genes of the viral RNA, are responsible for the coding of the respective antigens, the antigens occur at minimal changes in the structure (usually there is only one amino acid change ). This allows the immune system of the infected host, the "new" antigens difficult or impossible to detect, and the antibodies previously produced by the immune system can no longer bind the new antigen.

See also: Antigen original sin

Importance for the host

It is often possible to the immune system to combat some caused by antigenic drift new antigens with the old antibodies. But at some point no longer fit the antibodies, and the immune system must be set completely new to the antigens.

Until the immune system produced the new antibodies, the antigens may have changed again, so that only a small number can be turned off by viruses. In this way viruses can always evade the immune system, and there is a persistent ( permanent) infection.

Virological importance

Through antigenic drift is not immediately new subtypes of viruses form. This means that the antigens do not change completely. It is always changed only a small part of the gene and thus the associated antigen. The classification of the virus subtype on the basis of coat proteins thus remains the same. The emergence of a new subtype requires, among other an antigenic shift. However, it is conceivable that eventually forms by continuous antigenic drift, a new subtype.

Importance for the control of viruses

Vaccines usually contain a harmless, non-viable part of the original virus but containing antigens. The immune system produces antibodies in response to vaccination and can fight the virus invasion at once, before it comes to the outbreak of the disease. Through antigenic drift, the vaccines are mostly useless because the real virus has long since changed further by antigenic drift, until the vaccine is ready for use.

Sometimes, the time window is but large enough so that the use of a vaccine is worth, then at regular intervals ( yearly basis ) will be adapted to new antigens. This is, for example, in the conventional flu vaccine the case.

If the time window too small, the only possibility to control and prevent the spread mostly in quarantine or - in animals - killing of infected individuals.

Viruses in which antigenic drift occurs

• Influenza viruses
• Lentiviruses such as: HIV