Genetics of resistance to bacterial and parasitic infection / edited by D. Wakelin and J.M. Blackwell.

Date:
1988
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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Genetics of resistance to bacterial and parasitic infection / edited by D. Wakelin and J.M. Blackwell. Source: Wellcome Collection.

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    6 Genetics of Resistance to Bacterial and Parasitic Infection in unnatural hosts, remaining trapped in the epidermal layer. Many factors influence invasion of inner surfaces such as the intestinal mucosa and the respiratory tract. These include structure, physico-chemical characteristics, muco-ciliary mechanisms and, in the intestine, competition from other organisms. For many microparasites a major requirement for a successful invasion is the ability to attach to cellular surfaces; any factor which prevents or increases the efficiency of attachment will substantially influence the degree of establishment. In certain cases such attachment has a molecular basis (ligand- receptor), as discussed below. 1.4.2. Absence of appropriate receptors for attachment to and penetration of cell membranes Intracellular organisms have to recognize and attach to appropriate host cells before they can penetrate into them. It can be assumed that recognition has a molecular basis and is dependent upon the presence of complementary structures on cell and parasite. Even minor changes in either component will therefore be sufficient to prevent effective recognition. One of the best documented examples concerns the penetration of malarial parasites and particularly of Plasmodium vivax into mammalian red cells. The merozoites of this species require the presence of Duffy blood group antigens on the red cell surface for recognition; cells lacking these molecules are not invaded. In certain areas of West Africa the human population contains a high proportion of Duffy-negative individuals and the prevalence of P. vivax is correspondingly low. It is likely that similar phenomena occur with a number of other protozoal and bacterial microparasites, although the molecular basis is not always understood in comparable detail. Host complement and its activation is probably one of the factors relevant to the successful internalization of many species. Extracellular bacteria, particularly those which colonize mucosal surfaces, also demonstrate a specific recognition of cell membranes based upon receptor-ligand binding. In the case of the enteropathogenic K88-positive Escherichia coli, the presence of the specific receptor on host cells is genetically determined. Susceptible pigs possess these receptors, resistant pigs lack receptors, susceptibility being determined by autosomal-dominant genes. 1.4.3. Failure to survive after gaining access to the body because of naturally occurring parasiticida! factors The body resists invasion of many pathogens by producing a variety of factors capable of their destruction. These include gastric HCl, lysozyme and several serum components. Natural resistance against a number of protozoa (for example, trypanosome species) is expressed through the lytic action of serum factors such as high density lipoprotein or complement. Activation of complement by parasite surfaces is a particularly powerful mechanism of defence and one against which parasites have evolved counter measures such as the presence of thick extracellular coats of glycoprotein or carbohydrate.