Development of immune response markers in the trachea in the fetal period and the first year of life.

Immunoglobulin (Ig)-producing cells, T cells (CD3) and epithelial expression of secretory component (SC) and HLA class II determinants (DR, -DP, -DQ) were studied by immunohistochemistry in 16 fetal and 15 postnatal specimens from the tracheal wall. Small amounts of secretory component (SC) was present in the tracheal surface and gland epithelium in the fetal period and increased towards term. A few IgM-, IgD- and IgG-producing cells were present in some fetal specimens but no IgA- and IgE-producing cells were found. Only very few CD3+ T-cells were present in fetal specimens and intraepithelial T-cells were virtually absent until after birth. Premature infants that lived for 1 week had less SC epithelial expression than mature infants of the same age. The density of CD3+ T-cells, IgA-, IgM-producing cells as well as the epithelial SC expression increased rapidly after birth. Epithelial MHC class II expression was absent in fetal specimens. HLA-DR was detected on the apical border of the surface epithelium one week after birth and was extensively expressed throughout the remaining postnatal period. Epithelial DP and PQ expression were virtually absent during this same period. These features probably reflect local activation of the immune system in response to environmental factors.