Relatively few (less than 20) species of protozoa infect humans, but among these are four of the most formidable parasites of all, in terms of numbers affected and severity of disease: malaria, the African and American trypanosomes, and Leishmania (top left). These owe their success to combinations of the strategies found among bacteria and viruses: long-distance spread by insect vectors (compare plague, typhus, yellow fever), intracellular habitat (compare tuberculosis, viruses), antigenic variation (compare influenza) and immunosuppression (compare HIV). However, these strategies are so highly developed that complete acquired resistance to protozoal infections is quite exceptional, and what immunity there is often serves merely to keep parasite numbers down (‘premunition’) and the host alive, to the advantage of the parasite. The rationale for vaccination is correspondingly weak, especially because some of the symptoms of these diseases appear to be brought about by the immune response rather than the parasite itself.

In contrast, the intestinal protozoa (bottom left) generally cause fairly mild disease, except when immunity is deficient or suppressed. Nevertheless, together with the intestinal worm infections described on the next page, they add up to a tremendous health burden on the inhabitants of tropical countries.

African Trypanosomes 

Trypanosoma gambiense and T. rhodesiense, carried by tsetse flies, cause sleeping sickness in West and East Africa, respectively. The blood form, although susceptible to antibody and complement, survives by repeatedly replacing its surface coat of glycoprotein ‘variant antigen’ by a gene-switching mechanism; the number of variants is unknown but large (perhaps as many as 1000). High levels of non-specific IgM, including autoantibodies, coexist with suppressed antibody responses to other antigens such as vaccines; this may be due to polyclonal activation of B cells by a parasite product (compare bacterial lipopolysaccharides). Humans are resistant to the trypanosomes of rodents because of a normal serum factor (high-density lipoprotein [HDL]) that agglutinates them – a striking example of innate immunity.

Malaria 

Related posts:

Recognition and receptors: the keys to immunity The T-cell receptor Cell-mediated immune responses HIV and AIDS Immunodeficiency Immunostimulation and vaccination

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