This thesis reports on the infection by Trichuris muris of resistant (8ALB/c) and susceptible (AKR) strains of mice, here used as a model to determine the histological aspects of host-parasite interaction. The resistant BALB/c strain rejected the parasites after Day 14 post-infection. Sequential stages in the infection of the caecum were studied by LM; SEM and TEM. The structure of the crypts were examined in all stages from Day 0 (control) to adult stage (45 Days) of post-infection of AKR mice and from Day 0 (control) to Day 14 post-infection of BALB/c mice. The caecum tract is made up of four distinctive layers, the mucosa; the submucosa; the muscularis externa and the serosa. The mucosa is lined by columnar epithelium which is folded into numerous Crypts of Lieberkuhn. The crypts contain mucosal mast cells, enterocytes and goblet cells. The lamina propria contains some different cell types such as fibroblasts, and cells of the immune system. The early larvae were present in the caecal mucosa within the enterocytes at the base of the crypts and coiled towards the lumen. As the larvae developed, there was a corresponding increase in damage to the epithelium. In the late stages, as the worms matured in AKR mice, they expanded out of the crypt towards the lumen where they were visible near the surface. The worms were always surrounded by distorted syncytial tissue of the mucosa which forms a tunnel. In cross section, the pharyngeal region of the larvae showed an enormously elongated capillary-like oesophagus surrounded by an oesophageal gland (stichosome) which contains the stichocytes. The stichosome of the larvae at Day 14 post-infection of AKR mice contains two set of stichocytes, whilst in BALB/c mice the stichosome contains only one set of stichocytes. Also the larvae at Day 14 post-infection in BALB/c mice did not form tortuous tunnels on the surface of the mucosa. This is probably because the larvae in BALB/c mice do not develop as far as those in AKR mice. The cuticle of the larvae is a two-layered structure which appeared similar in both AKR and BALB/c mice. At Day 14 post-infection of both AKR & BALB/c mice, the larvae exhibited similar moulting activity. The oesophageal region of the adult worm within the syncytial tunnel is covered with a three-layered cuticle. The oesophageal region of the adult worm within the syncytial tunnel is covered with a three-layered cuticle. The oesophageal region of the larvae at Day 14 post-infection of both AKR & BALB/c contain a single cuticular pore area, the bacillary band which was at an early stage of its formation. The bacillary band of the adult faces the main body of the host's tissue. The bacillary band is an area of cuticular pores, each with underlying hypodermal gland cells and is a highly specialised region of the worm, probably with a secretory activity. The area beneath the cuticular pore forms the pore chamber. The cell membrane of the gland cells beneath the pore chamber is highly enfolded to form the lamellar apparatus. The adult stichosome is made up of a long column of 15-25 stichocytes. Cytoplasmic components of the stichocytes include granules, rough endoplasmic reticulum, lipid droplets, glycogen, mitochondria, Golgi complex and canalicular trees. The possible role of secretory activity in the bacillary band has obvious implications in initiating a host immune response and was investigated by EM immunocytochemistry in order to detect the binding sites of antibodies to Emuris specific proteins. The rabbit polyclonal IgG showed strong reactivity with the cuticle, stichocytes, bacillary band, and the host tissue. The polyclonal anti-human IFN-gamma also showed strong potential in detecting antigens in the cuticle, stichocyte and bacillary band, but it was not particularly effective in detecting antigens in the host tissue. The polyclonal antimurine IFN-gamma demonstrated weak staining in the worm tissue, although it had a stronger staining in the host tissue. Rat monoclonal IgM showed a relative potential in staining the cuticle, bacillary band, stichocyte, and host tissue. This thesis provides new knowledge about the early and late stages of post-infection. It also provides new knowledge about the possible secretory function of the bacillary band in invoking an Immune response.