Lymphocyte recirculation and the gut: the cellular basis of humoral immunity in the intestine.

During the last 15 years evidence has accumulated which shows that the B immunoblasts generated in the gut associated lymphoid tissue (GALT) in response to antigenic stimuli from the intestinal tract are discharged into the intestinal lymph. The lymph stream carries them to the blood but most of them soon extravasate in the gut (and some other mucosae) and develop into plasma cells which synthesise secretory immunoglobulins. In sheep, there is some evidence that these cells may be derived initially from a sub-set of small lymphocytes which circulate preferentially through the GALT, but the situation in rodents is less clear. However, in most species it is becoming clear that although some of the antibodies produced by the sub-mucosal plasma cells are secreted directly into the lumen of the gut many find their way, via the lymph, into the blood and, if in the form of polymeric IgA, they are rapidly and actively transported by the hepatocytes into the bile and thus gain direct access to the duodenum in concert with the entry of freshly ingested food and bacteria.