Rat hemolymph nodes provide a direct communication site for macrophages interacting with erythrocytes and mast cells.

Introduction: Hemolymph nodes, characterized by erythrocyte rosettes, are found in humans and animals, including rats. The cytoarchitectural differences they share with lymph nodes and the spleen are unknown. Herein, we describe the cytoarchitecture of rat hemolymph nodes.

Methods: We performed immunohistochemical analyses with antibodies against CD68, Iba-1, CD3, CD20, and S-100. Hematoxylin and eosin staining was used to compare findings with sections from ordinary lymph nodes and spleen.

Results: Hemolymph nodes exhibited erythrocyte rosettes with macrophages immunopositive for CD68, Iba-1, and CD3, which were rare in the physiologically normal spleen and lymph nodes. Additionally, sinusoidal macrophages often showed close apposition to erythrocytes and mast cells. Accumulation of cells immunoreactive to CD20, a B-lymphocyte marker, was seen only in the germinal centers of ordinary lymph nodes, not in the hemolymph nodes or spleen. Ordinary lymph nodes and spleen showed well-developed reticular configurations of cells with immunoreactivity for S-100, a marker for dendritic cells, unlike hemolymph nodes, suggesting less-developed antigen-presenting ability in the latter. Despite similarities to ordinary lymph nodes and spleen, the direct contact with erythrocytes and mast cells in hemolymph nodes suggests they facilitate direct cell-to-cell communication for macrophages, erythrocytes, and mast cells.

Conclusion: Our findings imply that the hemolymph nodes are a unique immune organ, differing from ordinary lymph nodes and spleen.