In small ruminants, the development of B cells differs from that in mice or in 
man. The anti-body repertoire is expanded in the Peyer's patches of the terminal 
ileum where each B-cell follicle is found by a few cells. To investigate the 
amount of founder clones in bovine ileal follicles, we have used sex mismatched 
cattle twins. These animals are chimeric due to placental anastomoses. 
Y-chromosome targeted in situ hybridization was used to trace donor-derived 
cells of the male genotype in a female recipient (called a freemartin). A strong 
clustering of lymphoid cells originating from either twin was seen in the ileal 
Peyer's patches (IPPs). Furthermore, the follicles displayed a low amount of 
immunoglobulin heavy chain gene configurations in comparison with the splenic or 
jejunal follicles. These findings strongly suggest that as in sheep, the B-cell 
follicles in cattle IPPs develop oligoclonally.