Selective visualization of human dendritic reticulum cells in reactive lymphoid follicles by the immunohistochemical demonstration of the subunit A of factor XIII (F-XIIIa).

Immunohistochemical techniques have been used to localize clotting factor XIII subunit A in human reactive lymphoid follicles. The follicular dendritic reticulum cells (DRCs) were identified by the monoclonal antibodies R4/23 and OKB-7 as well as by their 5'-nucleotidase positivity. Follicular histiocytic reticulum cells (HRCs) were demonstrated by their acid phosphatase and non-specific esterase reactions. Capillaries were selectively visualized by adenosine triphosphatase. The immunohistochemical demonstration of F-XIIIa was preferably carried out in combination with one or two of the above marker techniques, on the same cryostat section. The subunit A of factor XIII is present in follicular DRCs. Their selective immunohistochemical demonstration with antibody against F-XIIIa requires formaldehyde fixation of cryostat sections. Similar fixation, however, is inappropriate for the demonstration of F-XIIIa reactivity of DRCs in paraffin sections. For this purpose, acetic acid-formalin fixation is useful. Follicular HRCs are consistently negative for F-XIIIa, contrary to the F-XIIIa positivity of sinusoidal and interfollicular HRCs. Developmental and functional implications of F-XIIIa reactivity in DRCs and HRCs are suggested.