An update on antigenic variation in African trypanosomes

African trypanosomes can spend a long time in the blood of their mammalian host,where they are exposed to the immune system and are thought to take ad vantage of it to modulate their own numbers. Their major immunogenic protei n is the variant surface glycoprotein (VSG), the gene for which must be in one of tbe 20-40 specialized telomeric expression sites in order to be tran scribed. Trypanosomes escape antibody-mediated destruction through periodic changes of the expressed VSG gene from a repertoire of similar to 1000. Ho w do trypanosomes exclusively express only one VSG and how do they switch b etween them?