Unlike the Peyer's patches of rats and mice, which are considered to be 
secondary lymphoid organs, the ileal Peyer's patch of sheep is thought to be 
responsible for the primary generation of B cells, like the bursa of Fabricius 
of birds. The ileal Peyer's patch of sheep shows prenatal maturation, 
antigen-independent lymphopoiesis, a rate of lymphocyte production larger than 
that of the thymus, and involution at a young age. Follicles contain few T cells 
and have an IgM+, relatively immature B lymphocyte population, as judged by 
B-cell differentiation markers. The follicle-associated epithelium of the ileal 
Peyer's patch is of a special type that sheds carbonic anhydrase-rich, 
50-nanometer membrane-bounded particles (carbonic anhydrase-reactive particles; 
CAP) into the intercellular spaces. The CAP filter into the follicle centre and 
are taken up by lymphocytes. They represent the epithelial (bursa-like) element 
in an otherwise mesenchymal stroma of reticular cells embedding the follicle 
lymphocytes. Transepithelial transport of macromolecules, with the formation of 
multivesicular body-like cytoplasmic vacuoles, appears to be the basis for CAP 
formation. The jejunal Peyer's patches are devoid of CAP, persist in the adult 
animal, contain M cells with clusters of B cells in the follicle-associated 
epithelium, and have many CD4+ lymphocytes in the follicles and in the 
interfollicular areas. Aggregates of lymphoid follicles in the large intestine 
resemble the jejunal Peyer's patches with respect to their lymphocyte population 
and the ileal Peyer's patch with respect to their follicle-associated 
epithelium.