IMMUNOHISTOLOGIC DETECTION OF THE PRIMARY FOLLICLE (PF) IN HUMAN FETAL AND NEWBORN LYMPH-NODE ANLAGES

The primary follicle (PF) emerges as a globular nest of follicular den dritic cells (FDC) and lymphocytes in the lymph node anlage in the 16t h gestational week. It increases in size with age but no germinal cent er is found until several months later, after birth. Using a panel of monoclonal antibodies, the authors have defined phenotypes of componen t cells of the PF. The PF contains a B-cell population including IgM+, CD20+, CD21+, and CD24+ cells, together with a T-cell population incl uding CD3+, CD4+, CD5+, and CD8+ but no IgG+ cells. It also contains m any CD5+ B cells and several IgD+ and alkaline phosphatase-positive ce lls but few CD15+, CD25+, CD30+, CD38+, and Ki-67+ cells. CD24+ and de ndritic reticulum (DRC)-1+ cells show an irregular meshwork pattern in the PF. CD5+ B cells appear even before the formation of the PF and i ncrease after formation of the PF. The lymphocytic phenotype of the PF is similar to that of the mantle zone of the secondary follicle. The phenotypic characteristics indicate that the PF appears as an aggregat ion of CD5+ B cells and plays an important role as the ancestor of the secondary follicle as well as helper T cells and FDC.