The interbranchial lymphoid tissue (ILT) has recently been
described in the gills of salmon. It is located upon the caudal edge of
the interbranchial septum at the base of the gill filaments [1]. ILT is
formed by an accumulation of lymphoid cells, the majority being T
cells, supported by a meshwork of interstitial epithelial cells [2].
The expression of T-cell receptor (TCR), CD3ε and MHC class II
positive cells together with the organization of the T cells in the
center of the ILT and more flattened epithelial cells and mucous
cells in the periphery of the ILT indicate an organized lymphoid
tissue [1,3]. ILT has been considered part of the gill-associated
lymphoid tissue (GIALT), combined with dispersed leucocytes
within the lamellar epithelium [4]. However, being purely an
intraepithelial structure makes the ILT different to previously
described lymphoid tissues in mammals [2].