Haemocytic changes associated with the immune reaction of nematode-infected larvae of Orthellia caesarion

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Abstract
The immune reaction of larvae of O. caesarion against the nematode H. autumnalis is characterized by melanization of the cuticle of the parasite and the accumulation and fusion of host haemocytes to form a homogeneous, multinucleated capsule around the parasite. The immune reaction is manifested as a change in the haemocyte picture of the host long before there is any visible evidence of melanization and encapsulation. Differential haemocyte counts from host larvae at various stages of infection showed a decrease in the number of oenocytoids. These data, together with histological studies, indicate that oenocytoids play an important role in the melanization and encapsulation of the nematode. The apparent increase in the number of circulating haemocytes in parasitized larvae suggests that the haemocytes are stimulated to move out of areas where they are normally located and to react against the parasite.