Recent advances of tho research on the age-related alterations of cellular and molecular defense systems have been reviewed with special emphasis on its relationship to reticuloendothelial system.
Rapid progress of the research on the cytokine network and cellular interaction of defense systems began to clarify the precise mechanism of immunosenescence and to provide directions for therapeutic means in geriatrics.
The proportion of the elderly in the world is rapidly increasing and so is the urgent need to provide sophisticated health care for the elderly.
We must further intensify the research to strengthen the defense systems of the elderly, leading to provide measures for the longevity and quality of life of the elderly.

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The proliferation site of B lymphocytes in secondary lymphoid follicle of the tonsil was evaluated. Five follicles were selected to detect histon H3 mRNA as marker of cells in S-phase of the cell cycle by in situ hybridization. Moreover, 11 follicles were selected and stained immunohistochemically with serial frozen sections with cyclin B1 as a G2-M phase marker, p34cdc2 as a marker mainly of G2-M phase, cyclin E as a marker mainly of late G1 phase, and Ki-67 as a marker of cells in the cell cycle. Secondary lymphoid follicles were divided into five zones by the HE stain and the expression of CD23 (Fc ε receptor II) to calculate the positive rate of histon H3 mRNA and these proteins. The positive rates of histon H3 mRNA, cyclin B1, and p34cdc2 were the highest in the dark and the outer zones. The result implies that the outer zone as well as the dark zone is a proliferatation site of B cells.
Nextly, the phase of the cell cycle of B cells in the follicular dendritic cell (FDC)-lymphocytes clusters, which were obtained from the germinal center of the secondary lymphoid follicle of human tonsils, is evaluated. The rate of cyclin B1-positive lymphocytes in the cluster (4.6%) is lower than that of the lymphocytes out of the cluster in germinal center (27.7%) and, moreover, the rate of Ki-67-positive cells in the cluster (35.9%) is lower than that of cells out of clusters (61.5%). The result shows that few G2-M phase lymphocytes are retained in the FDC-lymphocytes clusters and support that the B lymphocytes actively do not proliferate in the clusters.

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Langerhans cells (LCs) in the squamous metaplastic epithelium of the trachea and the urinary bladder of vitamin A-deficient rats were examined. Both round, immature cells which were considered to be precursor LCs, and mature LCs with prominent dendritic processes, were recognized in the present study. Both cells were positive for anti-protein kinase type II antibody (PKC-II), and although the former did not have Birbeck granules (BGs), the latter did. The precursor LCs without BGs first appeared in the early stage of squamous metaplasia. As the differentiation of squamous metaplasia progressed, the number of BG bearing LCs increased, and the precursor cells became mature LCs in the later stage of squamous metaplasia of both the trachea and the urinary bladder. In the later stage of squamous metaplasia of the urinary bladder, LCs with BGs also appeared in the lamina propria. These findings indicate that precursor cells which probably migrate from the bone marrow, disperse into the metaplastic squamous epithelium in morphologically immature forms, and mature into BG bearing LCs as a result of contact with squamous epithelium.

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