Describe four actions of effector CD4 TH2 cells that provide protection from infections by intestinal helminths and lead to expulsion of the parasites from the gastrointestinal tract
What will be an ideal response?
(i) IL-13 secreted by TH2 cells enhances epithelial cell turnover and the sloughing of parasitized epithelial cells. IL-13 also stimulates goblet cells to produce mucus, which then interferes with adherence of helminths to mucosal surfaces (see Figure 10.27).
(ii) IL-5 secreted by TH2 cells attracts eosinophils and activates them to secrete major basic protein, which is cytotoxic to helminths. Eosinophils can also attack parasites directly when their Fc receptors are cross-linked by parasites that have become coated with antibody, especially IgE. In this situation the eosinophil pours out its toxic granule contents directly onto the parasite surface.
(iii) CD4 TFH cells engage antigen-specific B cells and influence the switch to the IgE isotype. The anti-parasite IgE antibodies produced then bind to high-affinity Fcε receptors on mast cells. Cross-linking of the bound IgE by parasite antigens activates the mast cells to release inflammatory mediators such as histamine. These mediators induce muscles spasms and watery feces in the gut that help to eliminate the parasite from the body.
(iv) IL-3 and IL-9 secreted by TH2 cells attract mast cells to the site of parasite infection.
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