ABSTRACT
Immune cells in elasmobranch fishes are produced in a variety of tissue and organ sites, some of which are common to higher vertebrate immune systems, whereas others are sites not found in any other vertebrate animal group. Thymus and spleen participate in elasmobranch lymphopoiesis in much the same way as they do in higher vertebrates, but in the absence of bony skeletons and lymphatic systems, sharks and their batoid relatives possess unique bone-marrow equivalent tissues that effectively compensate for the lack of marrow and lymphatic tissues. These unique lymphomyeloid tissues are the epigonal and Leydig organs, associated with the gonads and esophagus, respectively. These four tissues/organs constitute the primary sites for leukocyte production in the elasmobranch immune system.
