The previous sections illustrate that we are still defining (a) which sets of lymphoid cells are present in the intestine and which are not, (b) which sets are peculiar to the intestine, and (c) how the sets that are there function in the intestinal microenvironment. An understanding of the latter point is going to require knowledge of how these sets communicate with and regulate one another via cell surface molecules such as MHC class I and class II molecules, and via soluble mediators or lymphokines. The recent advances in various technologies make this a particularly exciting time in this field because the tools are now available to address and answer some of these basic and important questions in mucosal immunology. At the same time these advances hold great promise for our eventual understanding of chronic inflammatory diseases of the intestine. As was mentioned at the outset, the immune system has considerable power for both protection and destruction. It remains a puzzle how this latter potential is contained and controlled in the intestine of most individuals, such that they do not have inflammatory disease even in the setting of intense stimulation by substances, such as endotoxin, that are phlogistic elsewhere in the body. An answer to the question of why everyone does not have intestinal inflammation could provide new insights into the mechanisms involved in chronic intestinal inflammatory diseases. The recent advances just detailed, as well as others sure to come, suggest that it is only a matter of time before such questions are answered. © 1986.