بسم الله الرحمن الرحيم. Histology of the mucosal lymphatic tissues. The lymphoid system is divided into a series of functional anatomical compartments, of which the 2 most important are:
They comprise the linings of the gastro-intestinal, urogenital & respiratory tracts.
The mucosal surfaces of the body are highly vulnerable to infection. They are thin and permeable barriers to the interior of the body because of their physiological activities in gas exchange (the lungs), food absorption (the gut), sensory activities (eyes, nose, mouth, and throat), and reproduction (uterus and vagina).The Mucosal Immune System
The types and distribution of T cells differ, with significantly greater numbers of intraepithelial T lymphocytes (IEL) in and lamina propria (LPL) in the gut mucosa compared with peripheral L.Ns & blood.
The mucosal lymphoid system is exposed to foreign antigens from foods, commensal bacteria of gut & from pathogenic microorganisms and parasites. No immune response can normally be detected to food antigens.
Indeed, soluble antigens taken by mouth may induce antigen-specific tolerance or antigen-specific suppression. In contrast, pathogenic microorganisms induce strong protective response.
A) Mucoso-lymphoid organs
B) Diffusely distributed immune cells
The appendix is a short diverticulum arising from the blind terminus of the cecum. It has a narrow, irregular lumen, which often contains debris. The wall is thickened due to the presence of large aggregates of lymphoid nodules in the mucosa and even in submucosa (in middle aged and older individuals).
Simple columnar (contains surface columnar cells and goblet cells).
Lamina propria displays numerous lymphoid nodules (capped by M cells) and lymphoid cells. It does not form villi but possess shallow crypts with some goblet cells, surface columnar cells and numerous enteroendocrine cells.
Muscularis mucosa is composed of an inner circular and outer longitudinal layer of smooth muscle.
Submucosa is composed of fibroelastic C.T. containing confluent lymphoid nodules and associated cells.
In addition to the organized lymphoid tissue in which induction of immune responses occurs within the mucosal immune system, small foci of lymphocytes and plasma cells are scattered widely throughout the lamina propria of the gut wall. These represent the effector cells of the gut mucosal immune system.
M (Microfold) cells:
Site: they are specialized epithelial cells in the epithelium overlying the Peyer's patches. They are much less prominent than the absorptive gut epithelial cells (enterocytes).
Shape and structure: they have microfolds on their luminal surface, instead of the microvilli present on the absorptive epithelial cells of the intestine. M cells lack a thick surface glycocalyx and do not secrete mucus. Hence they are adapted to interact directly with molecules and particles within the lumen of the gut.B) Immune cells
Because M cells are much more accessible than enterocytes to particles within the gut, a number of pathogens target M cells to gain access to the subepithelial space, even though such pathogens then find themselves in the heart of the adaptive immune system of the intestine, the Peyer's patches.
Antigens are transported through M cells by the process of transcytosis and delivered directly to antigen-presenting cells and lymphocytes of the mucosal immune system.