The existence of a secretory immune system in the female genital tract has been demonstrated by the predominance of immunoglobulin (Ig)A-producing plasma cells in human fallopian tube, uterine cervix, and vagina. Epithelium lining fallopian tubes expresses a receptor for IgA, secretory component (SC), and thus resembles other secretory tissues such as intestine, mammary, lacrimal, and salivary glands. The present study extends the characterization of the local immune system in the fallopian tube and assesses its response to infection. We examined normal and infected fallopian tubes from surgical specimens, obtained at tubal ligation and abdominal hysterectomy, for the presence of Ig-producing cells, T cells, and natural killer cells. All tubular segments contained a predominance of IgA plasma cells in the subepithelial lamina propria. The epithelial cells were strongly positive for SC. Luminal contents stained positively for IgA, SC, and J chain, suggesting that this material contained secretory IgA. Submucosal plasma cells of IgM and IgG classes were less frequent than IgA. T cells were present in numbers approximately twofold greater than plasma cells in normal fallopian tubes. T-suppressor (CD8+) cells, which may function in the induction of immune tolerance, were present in the intraepithelial spaces. Infected segments of fallopian tubes demonstrated six- to tenfold increased numbers of plasma cells of all classes. These data suggest that a local immune system is functioning in the human fallopian tube and may provide a first line of defense against tubal infection and the prevention of tubal factor infertility.