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The smoltification of farmed Atlantic salmon is commonly associated with mild immunosuppression. However, B cells may deviate from this trend, showing increased proliferation and migration during this period. This study assessed the effects of smoltification and adaptation to seawater in a controlled experiment. Analyses were conducted on the head kidney, spleen, gill, and both visceral and subcutaneous fat (VAT, SAT) across four time points: parr, early and complete smoltification, and twelve weeks post-seawater transfer. Gene expression analysis was performed to track the distribution and developmental changes in their B cells. Expression profiles of three types of immunoglobulins (ig), including membrane-bound and secreted forms of igm, as well as B cell-specific markers pax1 and cd79, showed strong correlations and contrasted with profiles of other immune cell markers. The highest levels of expression were observed in the lymphatic tissue, followed by the VAT. Enhanced expression in the gill and adipose tissues of smolts suggested an increase in B cell populations. Parallel sequencing of the variable region of the IgM heavy chain was used to track B cell traffic, assessed by the co-occurrence of the most abundant sequences (clonotypes) across different tissues. Smoltification markedly enhanced traffic between all tissues, which returned to initial levels after twelve weeks in the sea. The preferred migration between the head kidney, spleen, and VAT supports the role of abdominal fat as a reservoir of lymphocytes. These findings are discussed in the context of recent studies that suggested the functional significance of B cell traffic in Atlantic salmon. Specifically, the migration of B cells expressing secreted immunoglobulins to virus-infected hearts has been identified as a key factor in the disease recovery and survival of fish challenged with salmon alphavirus (SAV); this process is accelerated by vaccination. Additionally, the study of melanized foci in the skeletal muscles revealed an association between antigen-dependent differentiation and the migration of B cells, indicating a transfer from local to systemic immune responses. Updating the antibody repertoire in the lymphatic and peripheral tissues of smolts may assist in their adaptation to the marine environment and in encountering new pathogens. Emerging evidence highlights B cell migration as an important and previously unrecognized immune mechanism in salmonids. [ABSTRACT FROM AUTHOR]

Type I allergy is mediated by the formation of IgE antibodies to proteins secreted by nonreplicating microorganisms (plant pollen, house dust mites, etc.) that enter the mucous membranes in very low concentrations. The mechanisms and localization of naive B cells’ switching to IgE production have not been fully determined. The aim of this work was to determine the switching site of B cells and the traffic of IgEproducing B cells in mice immunized with a low dose of equimolar mixture of egg proteins Gal d1, Gal d2, and Gal d3. Allergens in saline solution were injected into the withers of mice 9-10 times with an interval of 2-3 days; the total dose was 2.7 µg/mouse. The production of IgE to Gal d proteins in the blood and by B cells isolated from the withers, draining lymph nodes, spleen, and bone marrow of immune mice was analyzed in dynamics after cessation of sensitization. Both in blood and in in vitro cultures, the dominance of IgE changed from the recognition of the LMW Gal d2 during the sensitization of mice to the HMW Gal d3 after sensitization was discontinued. In this model, an IgG memory response appeared only a month after the end of sensitization and recognized only Gal d3. In vitro cultures showed that B cells switched to IgE production locally in the withers with low traffic to the spleen. In the blood serum, IgE titers for all Gal d proteins decreased after the cessation of sensitization and persisted for a long time. A month after the cancellation of the sensitization, a pool of B cells producing IgE in vitro appeared in the spleen. These B-cells died after 20-30 days as no in vitro IgE production was observed later than 85-90 days. The results obtained allowed us to draw several conclusions. B cells switch to IgE synthesis locally at the site of allergen injections. The response was two-phase: LMW Gal d2 was recognized in the early response, while HMW Gal d3 was recognized in the late phase. In this model, the IgG response to HMW Gal d3 was clearly dominant. In conclusion, it has been shown that when the immune system recognizes a mixture of proteins originating from some allergen, the dominance of proteins recognized by both IgE and IgG is observed. Since allergy patients most often do not have IgG antibodies, it can be assumed that in this case an acute phase response, supported by antigen intake, is observed, in which LMW allergens are recognized.