Several mechanisms have been proposed to explain the observation of antigenic variation at the molecular level. These are all based on the observation that only VSG genes located at, or near, a chromosome end (called a telomere) are expressed (this does not hold for metacyclic VSG genes). VSG genes make part of a large gene family but only a limited number of VSG genes are present in expression sites on telomeres. Only one expression site can be active at a given time, explaining why only one type of VSG is expressed at a time. The mechanisms of VSG activation proposed are shown in the figure below.
Continue