Respiratory tract-associated lymphoid tissue in conventionally raised sheep.

The distribution and morphological features of respiratory tract-associated lymphoid tissue was investigated in 20 clinically healthy sheep aged from 6 months to 9 years. Five different forms of lymphoid tissue were identified, but their distribution varied between regions of the respiratory tract and between individuals. Scattered lymphoid cells were the most predominant form. Dense lymphoid aggregations were frequently seen in the pharyngeal tonsil and bronchioles. They were less common in the mid-nasopharynx and about the opening of the auditory tube, occasionally seen in the anterior nasopharynx, trachea and bronchus and rarely found in the regions of the nasal conchae, nostril, nasal vestibule and epiglottis. Nodular lymphoid aggregations with morphologically distinct lympho-epithelium were seen only in the pharyngeal tonsil and opening of the auditory tube region. Small numbers of intra-epithelial lymphoid cells were always present throughout the mucosa, but intraluminal lymphocytes were observed mainly in the regions of pharyngeal tonsil, trachea, bronchi and large bronchioles. The respiratory tract-associated lymphoid tissue is less developed in sheep than in most other species studied, except perhaps cattle.