**7. Host-parasite interaction**

The relationship between *S. stercoralis* and its host is complex, and little is known about the immunomodulatory mechanisms that regulate this interaction. Different factors are involved, including the capacity of the parasite to replicate, the adequacy of the host immune response, and the ability of the parasite to evade those responses (Grove, 1994; Trajman et al., 1997).

#### **7.1 Cellular immune response**

During helminthic infection, Th2-type cell-dependent host defences that involve CD4 cells are developed (Maizels & Yazdanbakhsh, 2003; Anthony et al., 2007). In human hosts and animal models, the cellular immune response to *Strongyloides* infection is characterised by intraepithelial and tissue eosinophils, neutrophils and mast cells with Th2-type production of cytokines such as IL-4, IL-5 and IL-13. Conversely, Th1-type responses are downregulated during nematode infection (El-Malky et al., 2003; Paterson et al., 2008; Iriemenam et al., 2010).

### **7.1.1 Eosinophils**

Eosinophils are essential against nonphagocytosable parasites, such as *Strongyloides,* that cannot be ingested because of their large size. Eosinophils defend the host by attacking the parasite via the FcεRI receptor, capturing antigens from the worms and presenting the antigens to T cells to initiate an antigen-specific immune response (Galioto et al., 2006; Padigel et al., 2006; Iriemenam et al., 2010). Others mechanisms may be involved, including antibody-dependent cellular cytotoxicity (ADCC) mediated by eosinophils on the parasite surface, which releases toxic molecules in an attempt to eliminate the parasite (Ligas et al., 2003; Klion & Nutman, 2004).

#### **7.1.2 Mast cells**

Mast cells have an important role in the defence against *S. stercoralis* by inhibiting the invasion of the adult worm into the intestinal epithelium, promoting the stimulation of gut motility, mucus release and expulsion of the parasites. In addition, mast cells induce the attraction and modulation of eosinophils (Kobayashi et al., 1998; Concha et al., 2005).
