An overview of intestinal immunity and malabsorption.

Intestinal immune responses are adapted to function at external mucosal surfaces. Specialized forms of antibody, secretory immunoglobulin A (IgA) and immunoglobulin M (IgM), provid humoral immunity but little is known of local cell mediated immune reactions. Antigens in the intestinal lumen gain preferential access via Peyer's patches in which sensitised lymphocytes proliferate before entering the lymphatic system. These lymphoblasts return to the intestinal mucosa via the bloodstream to provide predominantly IgA antibody responses. Secretory IgA antibody can neutralize viruses, bacteria and toxins, and appears to block the entry of some food antigens into the lamina propria. Disturbances of intestinal immunity may result in malabsorption. Immunodeficiency states are often associated with malabsorption due to Giardia lamblia infestation. In alpha chain disease there is a malignant expansion of plasma cells in the intestinal mucosa which secrete an abnormal heavy chain fragment of IgA. Arthus type hypersensitivity reactions to milk proteins and gluten may contribute to the mucosal injury in patients suffering from milk allergy and coeliac disease.