Granulocytes are generally considered effector cells of the innate immune response (46).

The importance of each of these cell types (i.e. RCs, MCs and neutrophils) therefore is worth considering in the context of the current study. Recent studies on both wild and farmed fish suggest that RCs represent an immune cell type closely linked to other piscine inflammatory cells (45,47). RCs are found exclusively in fish in a wide range of tissues and are commonly associated with epithelia (23). As M. wageneri destroys the epithelia at the site of attachment, it was not possible to compare the number of RCs in uninfected and parasitized tench. The presence of RCs in the intestinal submucosa of infected tench and those in direct contact with the blood vessels is interesting and suggests that

RCs also use the circulatory system to migrate to the site of infection. Similar findings have been reported ALK phosphorylation for fish that were infected with acanthocephalans (10,48). Fish MCs, also known as eosinophilic granule cells, have cytochemical features, functional properties and tissue locations that have led to the suggestion that they are analogous to mammalian MCs (22,23,25). Several published reports on the intratissue migratory nature of MCs suggest that fish may have two populations of MCs, one circulating and one resident, and that the presence of parasites induces the recruitment of MCs to the site of infection (25,28). The significantly higher number of MCs found at the site of parasite attachment, CYC202 price when compared to uninfected tench, in MycoClean Mycoplasma Removal Kit the current study supports similar results reported for other fish–helminth systems (48). In teleosts, considerable descriptive data exist showing how MCs degranulate in response to a variety of known degranulating agents (49) and pathogens (23,25,30). In parasitized tench, an intense degranulation of MCs was seen at the site of tapeworm infection, notably in the immediate zone surrounding the scolex.

It is likely that the secretions produced by the MCs may have a role in attracting other cell types (i.e. neutrophils) involved in the inflammatory process, particularly during the period of initial pathogen challenge (24,32). One study reported that intra-epithelial MCs are present in low numbers in healthy epithelium but then dramatically increase in number with certain parasitic infections (50). In the current study, MCs, in the intestines of parasitized tench, were frequently observed among epithelial cells. Neutrophils are among the first cell types to arrive at the sites of inflammation and play a critical role in the teleost innate immune defence system (31). In infected tench, numerous neutrophils were observed to co-occur with MCs in the submucosa at the sites of M. wageneri attachment. A similar observation was found in the livers of minnows, Phoxinus phoxinus (L.