This is due to antigenic variation. After 5 to 7 days, these antibodies destroy most of the trypanosomes, and the parasitemia decreases, but a new wave of trypanosomes appears. These are unaffected by antibodies generated against the previous wave, so the immnune system has to start over again. This process continues essentially indefinitely, until the death of the host. During this time, we continue to make primary responses against new antigens.
M go sky high, to no useful effect.
The first clue came from electron microscopy of trypanosomes. In 1979, Keith Vickerman drew attention to a feature of the surface of infective forms of Trypanosoma brucei. This was the presence of a Surface Coat , a regular dense structure that surrounded the entire cell body and flagellum of the trypanosome. Vickerman suggested that the Surface Coat might contain the antigens responsible for antigenic variation. The picture shows a thin slice through a trypanosome viewed in an electron microscope.