Amyloid AA protein. Cellular distribution and appearance.

To investigate the cellular distribution and possibly the cellular origin of the AA protein associated primarily with the fibrils of secondary amyloid deposits, a variety of human tissues was studied by indirect immunofluorescence using a specific anti-AA immunoglobulin and FITC-goat anti-rabbit IgG. Positive cells were most frequently identified in reactive lymphoid tissues. They were most numerous in tonsils and, to a lesser extent, frequent in spleens. Both cellular appearance and distribution of the positive cells suggested a plasmacytic nature. Double staining with rhodamine-labeled anti-IgG demonstrated IgG within some of these cells. The data suggest that the AA protein may be an immunoprotein. In the absence of kinetic data, however, it is not possible definitively to assign the origin of the AA protein to the plasma cell.