Hiding in plain sight: A new lymphoid organ discovered in zebrafish

A small, unassuming, fleshy lobe at the base of the pectoral fin of fish has long been overlooked by scientists. In this issue of JEM, Castranova et al. (https://doi.org/10.1084/jem.20241435) describe this structure as a new secondary lymphoid organ. Translucent and externally located, the axillary lymphoid organ (ALO) shares striking structural similarities with mammalian secondary lymphoid organs. Due to its position and optical accessibility, the zebrafish ALO permitted noninvasive, high-resolution imaging of immune cell dynamics in live animals. These studies revealed likely interactions between T, B, and macrophage cells, arguing that the ALO may function in adaptive immune cell activation and provide a nexus for immune cell trafficking and communication.