Sows were immunized orally with live Escherichia coli according to various immunization schedules. Six pregnant gilts were used; 4 immunized at various intervals during the last month of gestation, 1 control immunized after parturition following suppression of lactation by weaning and 1 non-immunized control. The effect of oral vaccination on cell populations from lymphoid organs was studied. The in vitro proliferative responses of the cell populations to K88 antigen, anti-Ig sera and mitogens were used to demonstrate the distribution of sensitized lymphocytes over different lymphoid organs. The capacity of these cells to produce antigen-specific Ig was determined by in ovo translation of their mRNA.\ud \ud Oral administration of antigen resulted in the appearance of K88-positive cells in lymphoid organs. In lactating sows, sensitized cells preferentially occured in the mammary lymph nodes, whereas after suppression of lactation such a distribution was not seen. A possible route of migration of sensitized lymphocytes is discussed in relation to the local immune response. The antibody isotype produced by sensitized lymphocytes seemed to depend on the immunization schedule. The most effective schedule was one starting early in gestation and comprising frequent administration of antigen. This caused an optimal distribution of sensitized lymphocytes capable of IgA productio
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