 The body has several physical defense mechanisms against parasites including the continual sloughing of the gut epithelium to prevent parasite attachment.However, once an infection has occurred, the host's immune system attempts to limit the damage caused by the worm.Apart from the importance of the extrinsic factors of weather, climate and grazing management, the immune status of cattle is perhaps the most significant of all host factors influencing infection with "O. ostertagi".