Cellular and humoral elements of the lower respiratory tract of sheep. Immunological examination of cells and fluid obtained by bronchoalveolar lavage of normal lungs.

Bronchoalveolar washings were harvested from the excised lungs of 68 normal sheep of 3 different age groups (A: birth to 8 weeks; B: 6 months to 2 years; C: older than 2 years). Cells and fluid obtained were examined quantitatively and functionally. Fewer cells were present in the lavage fluids of Group A sheep compared with those of Groups B and C. Macrophages were the predominant cell type (70-80%) in all sheep, with lymphocytes being second in number. The lower proportion of lymphocytes in young sheep (Group A) was attributable to lack of B-lymphocytes. As a proportion of the lymphocyte population T-cells were in the majority (60-80%) in all sheep. The proportion of null cells was higher in young sheep than in adults. Pulmonary lymphocytes from sheep of all ages responded poorly to stimulation by the non-specific mitogens phytohaemagglutinin, concanavalin A, pokeweed mitogen and lipopolysaccharide of Escherichia coli. The proportion of neutrophils was higher in sheep over 2 years old compared with younger animals. Eosinophils were present in all age groups but their proportion was greatly increased in animals over 2 years of age. Basophils were absent in young sheep and were present in only low numbers in the lungs of adult animals. In young sheep (Group A) IgG was the predominant immunoglobulin. With age, the percentage of IgG in lung fluid decreased while that of IgA increased so that IgA became the predominant immunoglobulin in older animals (Group C). In sheep of all ages IgM was present in negligible amounts. The highest value of complement (C3) occurred in adult sheep (Group B). The total white blood cell counts and differential blood counts of all the sheep were within accepted normal ranges. As in the lungs, there was an age-associated reduction in the proportions of blood T-lymphocytes and an increase in B-lymphocytes. In contrast to lung lymphocytes, those from the blood of sheep of all ages exhibited a wide range of responses to mitogens. The lowest stimulation indices were observed in the oldest sheep (Group C). The results provide background data against which assessment can be made of changes consequent upon infection with respiratory pathogens.