Development of Defense Mechanisms in the Digestive Tract of the Chick

Abstract

The intestinal lumen contains food and bacteria in close contact with the mucus layer, which lines the absorptive surface. This matrix contains IgA and other antibacterial substances and acts as a barrier to harmful components and organisms. The underlying epithelial layer is protected by intraepithelial lymphocytes at the junctions between enterocytes and by the action of IgA within the enterocytes. The luminal contents are continuously sampled by M cells and dendritic cells in lymphoid tissue, which then present antigens to the complete array of immune cells present in the lamina propria, which can then mount appropriate defenses.This system is immature at hatch with the intestinal lymphoid tissues developing in the first posthatch days, mainly in the distal intestines. Lymphocyte colonization begins in the first posthatch week, whereas functional T lymphocytes and the presence of IgA secreting cells are only observed in the second posthatch week, and maturation occurs later. Thus, the hatchling appears to be susceptible to pathogens in the immediate posthatch period.