Bacterial infections and vaccines.

Abstract

We are exposed to multiple pathogens in our lifetime that can localize to body surfaces, spread systemically or both. The characteristics of an individual pathogen influences the type of effective immunity required to control it. For instance, to control helminth infections a pre-dominantly Th2 response is required, whereas control of intracellular pathogens such as Salmonella largely requires a Th1 response. Therefore, upon infection the host mounts an immune response tailored to the given pathogen. The nature and persistence of this response is predictive of the host’s ability to control the infection. Nevertheless, the accumulated impact of exposure to multiple pathogens and the subsequent impact of related activity can have a marked impact on immune homeostasis and function, to the point where the host can have increased susceptibility to additional unrelated infections. This altered immune homeostasis can be due to a combination of one or more factors that include: (i) perturbed barrier function; (ii) ineffective immune cell generation; (iii) defective immune cell trafficking; (iv) inappropriate adaptive cell polarization and (v) impaired cell function. Whichever of these apply, ultimately the impact on the host is often expressed as impaired control of an invading pathogen. Despite the impact multiple infections can have on the host, the relationship between infectious history and immune function after infection and vaccination remains understudied, even though they have a staggering social and economic impact. The reasons for this are manifold but include the importance of studying the infectious process of a single pathogen, the availability of appropriate tools and model organisms to uncover mechanisms. As our understanding of infections caused by individual pathogens increases, it is possible to broaden our focus to look at how distinct infections interact. In this chapter we wish to highlight some advances in our understanding of how infectious history can influence immunity to unrelated pathogens. For this we have focused on the influence of helminths on bacterial infections and immunity to vaccination.