Protozoan and Helminth Parasites
For protozoan parasites, the activation of TH1 cells will lead to parasite destruction. Helminth parasites usually cause the induction of T-helper 2 (TH2) cells. TH2 cytokines are necessary for clearance of parasites from the gut only. Protozoan antigens induce the production of interleukin-12 (IL-12). IL-12 directly stimulates T-cells and NK cells to produce interferon-γ (IFN-γ). IFN-γ leads to disease resolution because it can activate macrophages (Figure 1). An activated macrophage is capable of releasing reactive oxygenated species (ROS), nitric oxide, and other potent chemicals that kill the pathogen on site. This chemical process is collectively known as oxidative burst.
Helminth antigens do not induce the production of IL-12; instead, IL-4 is produced. IL-4, in turn, activates TH2 cells to produce IL-10; IL-10 further inhibits the production of IL-12 via a negative-feedback mechanism, in addition to other cytokines involved in a proinflammatory response (Figure 2). Recall that TH1 cells are involved in cell-mediated immunity while TH2 cells are involved in inflammation. As a result, IL-12 and IL-4 are key to determining immune responses to parasitic invasion.
Infection with helminths result in high levels of immunoglobulin E (IgE) and eosinophils induced by TH2 cytokines. This makes sense because we just illustrated that when the IL-12 is inhibited, an alternate cytokine IL-4 is produced. Another effect of helminth infection is mast cell hyperplasia. IL-4 is thought to come from NK1.1 and T cells. There is evidence that there is also an IL-4 independent pathway that initiates TH2 development that is driven by IL-13.
The following is a list of parasites that have been profiled:
click the animation to read more about the featured organism