THE PREVALENCE AND AGE DISTRIBUTION OF HUMAN THYMIC B-LYMPHOID FOLLICLES

The reported prevalence of B lymphoid follicles in the normal human th ymus differs widely between studies. In this histological study we hav e estimated the prevalence of these follicles, allowing for two factor s which we believe may be largely responsible for such differences. On e is the effect of stress of more than 48 hours duration, which has be en shown to be associated with a decrease in prevalence, and the other is the considerable differences in prevalence found in different age groups. The 639 thymuses in this study were selected from individuals who died less than 48 hours after the onset of their fatal condition. No individuals with autoimmune diseases were included. The study group was divided into age groups to demonstrate the age related variations in prevalence. The follicles were identified by their morphology and by their reactivity with the B lymphoid cell marker L26. No follicles were found in the thymuses of infants in the first year of life. Howev er the prevalence rose rapidly thereafter to a peak of 93% in individu als the first half of the second decade then gradually declined to onl y 9% in individuals older than 60 years. (The prevalence in young adul ts being approximately equal to that reported in young adults with aut oimmune diseases.) It seems likely therefore that a considerable amoun t: of the variation in prevalence found in previous studies is related to the inclusion in those studies of thymuses modified by stress as w ell as the grouping together of different age groups with differing na turally occurring prevalences of B lymphoid follicles. We concluded th at B lymphoid follicles occur frequently in the thymuses of healthy in dividuals, that their prevalence varies considerably with age and that their mere presence should not be accorded any pathological significa nce.