Trypanosome surface antigen (VSG) switching
Sleeping sickness in humans (and a related disease in cow) is caused by a unicellular parasite called trypanosome, which alternates between two hosts, tsetse fly and a mammal (Fig. 37.19). Trypanosome undergoes important biochemical changes involving diversity in the variable surface glycoprotein (VSG), a major component of surface coat, which provides antigenic reaction. Although, at any one time a trypanosome expresses only one VSG, its ability to change into any of the ~100 possible different VSGs is the secret of parasite's survival during fly-mammal cycle.

A tsetse fly, during a bite, gains the parasite, which loses its VSG but re-acquires a new VSG after three weeks when, it differentiates in 'metacyclic form'. This form enters mammalian bloodstream during a bite, where VSG keeps on changing every 1-2 weeks, so that the immune response always lags behind the change in VSG and the parasite evades the immune surveillance, perpetuating indefinitely, till it enters the central nervous system and often causes death.