Complement activation in the follicular light zone of human lymphoid tissues

A comparative immunohistochemical study of the distribution pattern of complement components and regulatory proteins within secondary lymphoid follicles was performed by the immunoperoxidase technique. Fifteen lymphoid tissues including appendices. Peyer's patches and tonsils were analysed. Sixty secondary lymphoid follicles with evident polarity, that is, the distinct coexistence of a light zone, dark zone and mantle zone in the same lymphoid follicle, were tested with single antibodies. The light zones were consistently immunostained in a dendritic meshwork pattern with all antibodies. The immunostaining patterns were classified into two major groups based on the immunoreactivity of the dark zone. One immunostaining pattern was characterized by no immunostaining of the dark zone to the majority of the antigens. The second group was characterized by a diffusely weak to moderate dendritic meshwork pattern of the dark zone to some of the immunostainings of C9 (monoclonal), S-protein, and DF-DRC1, and all immunostainings of CR1 (CD35), Ber-Mac-DRC (CD35), CR2 (CD21), and R4/23. All four complement regulatory proteins were localized by immunoelectron microscopy attached to the cell surface of the cells, including follicular dendritic cells, in the light zone. Our data indicate that there is an evident functional difference between the light zone and the dark zone, and that complete activation of the complement system occurs only in the light zone.