The immune response of the adipose tissue (AT) has been neglected in most animal models until recently, after investigations in human and mice linked obesity to chronic inflammation, highlighting the immune nature of this tissue. Thus, in mammals, the AT is increasingly seen as an active immune site, playing important roles in systemic and peritoneal responses. Despite this, in teleost fish, only a few studies have addressed the immune role of the AT. These studies revealed significant changes in the levels of transcription of a wide range of immune factors produced by the AT in response to diverse intraperitoneally delivered stimuli, and demonstrated the presence of B cells within the tissue . In the current study, we have continued the characterization of the immune role of rainbow trout (Oncorhynchus mykiss) AT, focusing on how the different B cell populations, that are present in the AT, react to an intraperitoneal stimulation . Initially, the B cell populations present in this immune tissue were characterized in comparison to B cells from other sources . As occurs in other rainbow trout tissues, IgM+IgD+ , IgM+IgD- and IgD+IgM - B cell subsets were identified in the AT. Interestingly, AT IgM+IgD- B cells showed a transcriptional profile that seems to correspond to cells that have already committed to plasmablasts/ plasma cells, being this profile much more pronounced than that of blood IgM+IgD- B cells . Consequently, the IgM -secreting capacity of AT cells is significantly higher than that of blood B cells. Additionally, we established how these B cell subsets responded when rainbow trout were intraperitoneally injected with a model thymus independent antigen, TNP-LPS. Our results demonstrate that AT B cell differentiate to plasmablasts/ plasma cells that secrete specific IgMs against the antigen, as happens in the peritoneal cavity, the spleen, the head kidney and in peripheral blood. Although the presence of these antigen-specific IgM secreting cells was more abundant in the peritoneal cavity, t hese differentiated B cells were detected in the AT for long time periods at levels similar to those of spleen and head kidney . Our results provide new evidence regarding the immune role of the teleost AT, demonstrating that it functions as a secondary lymphoid organ that promotes immunity to peritoneal antigens.