The lymphoid substance of the chicken's harderian gland is organized in two histologically distinct compartments.

Light and electron microscopical investigations revealed that the lymphoid structure of the chicken Harderian gland is organized in different histological frameworks. In the head the surface epithelium of the central canal can be classified as a lymphoepithelial tissue which covers the dense lymphoid substance. It consists of small and medium-sized lymphocytes, dendritic-like cells, and occasional macrophages. High endothelial venules are associated with intense lymphocyte migration and homing that gives circumstantial evidence for a T-dependent region, as found in a secondary lymphoid organ. The B-dependent germinal centers are also common structural units of the head region's lymphoid substance. The body of the gland is loaded with plasma cells of different maturation stages. They immigrate into the epithelium of the central canal and produce IgM and IgA. Only a few scattered IgG producing plasma cells can be found in the gland of Harder. This plasmocytic region accounts for the immunosurveillance on the conjunctiva and in the upper respiratory tract through antibody production against bacterial or parasitic infections. In both the head and body regions of the gland, anti-B-L (anti-Ia) antibody recognized scattered elongated cells which might represent dendritic cells. The immunological relationship between the two histologically different parts of the Harderian gland is unknown, but we speculate that the dense lymphoid tissue with high endothelial venule receives the blood-borne, immunologically mature, but uncommitted B cells. By the influence of local antigen stimulus, these B cells transform to plasma cells which gradually appear in the body of the gland. The lymphoid structures of the head and the body fulfill the function of secondary and tertiary lymphoid organs, respectively.