The prevalence and age distribution of human thymic B lymphoid follicles

The reported prevalence of B lymphoid follicles in the normal human thymus differs widely between studies. In this histological study we have estimated the prevalence of these follicles, allowing for two factors which we believe may be largely responsible for such differences. One is the effect of stress of more than 48 hours duration, which has been 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. However the prevalence rose rapidly thereafter to a peak of 93% in individuals the first half of the second decade then gradually declined to only 9% in individuals older than 60 years. (The prevalence in young adults being approximately equal to that reported in young adults with autoimmune diseases.) It seems likely therefore that a considerable amount of the variation in prevalence found in previous studies is related to the inclusion in those studies of thymuses modified by stress as well as the grouping together of different age groups with differing naturally occurring prevalences of B lymphoid follicles. We concluded that B lymphoid follicles occur frequently in the thymuses of healthy individuals, that their prevalence varies considerably with age and that their mere presence should not be accorded any pathological significance.