Mucosal Immune Defences Flashcards
What are mucosal surfaces?
Mucosal surfaces (mucosae) are predominately out of sight Continually bathed in layer of thick fluid that they secrete = mucus.
Mucus contains glycoproteins, proteoglycans, peptides, enzymes which help protect epithelial cells from damage and limit infection.
Mucosal surfaces comprise epithelial cell layer lining the gastrointestinal tract, respiratory tract, urogenital tract. Epithelial cells form a monolayer to make mucosal surfaces.
Also present in exocrine glands associated with pancreas, conjunctivae and lacrimal glands of eye, salivary glands, mammary glands of lactating breast.
Sites of communication – material and information passes between body and environment.
Functions of gas exchange, food absorption, sensory activity and reproduction require surfaces to be dynamic, thin, permeable barriers- Renders them vulnerable to subversion and breach by pathogens
How do mucosal surfaces defend themselves?
Mucosal surfaces have a combined surface area of ~400 m2
(about 200X that of skin). Covered largely by monolayer of epithelial cells.
Very large numbers of Ab-producing cells (exceed those of spleen,
lymph nodes and bone marrow combined)-
Secretion of 5-15 g Ab onto mucosal surfaces/day.
Expending a lot of energy to make so much mucosal antibody.
What is the problem with defending the GI tract?
Continuous contact with large populations of commensal microorganisms as well as large quantities of “foreign” material (food).
Challenge – how to make immune responses that eliminate pathogenic microorganisms, limit the growth and location of commensals, but do not attack food.
Make immune response that doesn’t act too much/inappropriately.
What are secondary lymphoid tissues?
Secondary lymphoid tissues and immune cells spread throughout gut and other mucosal tissues.
Present in surface epithelium and in underlying connective tissues = lamina propria.
Mesenteric lymph nodes defend gut = largest nodes in body arranged in a chain within gut connective tissue.
Gut associated lymphoid tissues – GALT – has B and T cell zones like other secondary lymphoid tissues.
Adaptive immune responses initiated locally in mucosa.
Guarding entrance to gut are palatine tonsils, adenoids, and lingual tonsils = large aggregates of secondary lymphoid tissue covered by layer of squamous epithelium – form Waldeyer’s ring.
Have all machinery needed to have response at mucosal surfaces.
What are Peyer’s patches?
Characteristic secondary lymphoid organs of intestine are Peyer’s patches.
Dome-like aggregates of lymphocytes of varying size.
5-200 B cell follicles with germinal centres, T cell areas, and dendritic cells- i.e. complete cellular machinery for wide range of immunological responses.
Also isolated lymphoid follicles – mostly B cells.
Both types of lymphoid structure overlaid by specialized epithelial cells called microfold (M) cells
(Have villi-free areas.
Have many peyer’s patches down gut- numbers lower with age.
These are free from villi.
Also have dendritic cells in here.
Follicles are separate, contain mostly B cells.
M cells= microfold cells.)
What are M cells?
Called M cause of microfolds.
Have no villi or microvilli.
Take up small amount of material in gut lumen- usually antigenic stuff.
Characteristic ruffles or microfolds on surface.
Absence of microvilli.
Unlike enterocytes, M cells:
-do not secrete digestive enzymes or mucus.
-lack thick surface glycocalyx.
-have weak system of lysosomes.
Take up intact microorganisms and particulate antigens from gut lumen. Transport them to Peyer’s patch to initiate adaptive immune response.
How is antigen sampled in the gut?
Healthy gut epithelium actively monitors the contents of the healthy gut lumen. This enables appropriate adaptive immune responses to be made against invading pathogens.
M cells of Peyer’s patches and isolated lymphoid follicles can uptake pathogens from lumen and transcytose them across epithelium into lymphiod tissues below- continuous sampling.
Dendritic cells resident in lamina propria can also capture pathogens independently of M cells.
