http://www.sciam.com/askexpert_question.cfm?articleID=000CAE56-7201-1C71-9EB7809EC588F2D7
What is the function of the human appendix? Did it once have a purpose that has since been lost? Ross Rowland Great Falls, Va.
Loren G. Martin, professor of physiology at Oklahoma State University, replies: "For years, the appendix was credited with very little physiological function. We now know, however, that the appendix serves an important role in the fetus and in young adults. Endocrine cells appear in the appendix of the human fetus at around the 11th week of development. These endocrine cells of the fetal appendix have been shown to produce various biogenic amines and peptide hormones, compounds that assist with various biological control (homeostatic) mechanisms. There had been little prior evidence of this or any other role of the appendix in animal research, because the appendix does not exist in domestic mammals.
"Among adult humans, the appendix is now thought to be involved primarily in immune functions. Lymphoid tissue begins to accumulate in the appendix shortly after birth and reaches a peak between the second and third decades of life, decreasing rapidly thereafter and practically disappearing after the age of 60. During the early years of development, however, the appendix has been shown to function as a lymphoid organ, assisting with the maturation of B lymphocytes (one variety of white blood cell) and in the production of the class of antibodies known as immunoglobulin A (IgA) antibodies. Researchers have also shown that the appendix is involved in the production of molecules that help to direct the movement of lymphocytes to various other locations in the body. "In this context, the function of the appendix appears to be to expose white blood cells to the wide variety of antigens, or foreign substances, present in the gastrointestinal tract. Thus, the appendix probably helps to suppress potentially destructive humoral (blood- and lymph-borne) antibody responses while promoting local immunity. The appendix--like the tiny structures called Peyer's patches in other areas of the gastrointestinal tract--takes up antigens from the contents of the intestines and reacts to these contents. This local immune system plays a vital role in the physiological immune response and in the control of food, drug, microbial or viral antigens. The connection between these local immune reactions and inflammatory bowel diseases, as well as autoimmune reactions in which the individual's own tissues are attacked by the immune system, is currently under investigation.
"In the past, the appendix was often routinely removed and discarded during other abdominal surgeries to prevent any possibility of a later attack of appendicitis; the appendix is now spared in case it is needed later for reconstructive surgery if the urinary bladder is removed. In such surgery, a section of the intestine is formed into a replacement bladder, and the appendix is used to re-create a 'sphincter muscle' so that the patient remains continent (able to retain urine). In addition, the appendix has been successfully fashioned into a makeshift replacement for a diseased ureter, allowing urine to flow from the kidneys to the bladder. As a result, the appendix, once regarded as a nonfunctional tissue, is now regarded as an important 'back-up' that can be used in a variety of reconstructive surgical techniques. It is no longer routinely removed and discarded if it is healthy.