Age-related changes in the morphology and the distribution of IgA and IgG in the palatine tonsils of yaks (Bos grunniens).

This study aimed to describe the age-related morphological changes and the distribution of IgA and IgG antibody-secreting cells (ASCs) in yaks. The palatine tonsils of twenty clinically healthy yaks, viz. newborn juvenile, adult and aged, were studied using histology, immunohistochemistry, and enzyme-linked immunosorbent assay (ELISA). The results showed that the palatine tonsils found in two tonsillar sinus were elongated kidney-shaped structures. Some external crypts and internal crypts were present. The palatine tonsils were partially enclosed by a connective tissue capsule and had trabeculae extending into the organ. Within these encapsulated organs, mucous glands were seen very obviously. Each crypt was highly branched and lined with stratified squamous non-keratinized epithelium. Several nonepithelial cells infiltrated between the epithelial cells, forming patches of reticular epithelium or lymphoepithelium. In newborn yaks, the lymphoid follicles were not observed. In other groups, the lymphoid follicles consisted of primary lymphoid follicles and secondary lymphoid follicles. Both IgA and IgG ASCs were distributed in the interfollicular areas, lymphoid follicles, the subepithelial areas of the non-reticular crypt epithelium, and the reticular crypt epithelium, with a few positive cells aggregated around the gland. The density of the two ASCs and the expression of the two proteins gradually increased from newborn to adult and reached a peak at adult age; they then decreased with age. However, the density of the IgG ASCs and the expression of IgG protein was significantly higher than that of IgA in all groups (P<0.01). The results indicated that the palatine tonsils were not only lymphoepithelial structures but also typical secondary lymphoid organs. IgG could be a significant component of mucosal immune responses in the palatine tonsils of yaks.