Simple SummaryThe immune response to pathogens is one of the organic functions at risk due to climatic change. The effects of a short-term increase in temperature on immune cell functions have been evaluated using a simple animal model, such as the sea urchin, exposed to temperatures above its comfort zone (17 °C), causing moderate and severe heat stress. After 3 and 7 days of exposure to the three temperatures tested, the coelomic fluid of the animals was collected. The coelomocytes were isolated and typed and the mitochondrial activity, the amount of lipid peroxidation, and the production of hydrogen peroxide were evaluated. These cells were also stimulated with phorbol 12-myristate 13-acetate (PMA) to evoke respiratory burst, a marker of immune response. Heat stress altered both the distribution of the various types of coelomocytes and their mitochondrial activity; it also increased lipid peroxidation and reduced the production of hydrogen peroxide. The respiratory burst occurred in the coelomocytes of all three experimental groups; however, it was greater in the group kept at 17 °C. In conclusion, thermal increase alters immune cell functions with possible repercussions on defense mechanisms against pathogens.AbstractDue to global warming, animals are experiencing heat stress (HS), affecting many organic functions and species’ survival. In this line, some characteristics of immune cells in sea urchins subjected to short-term HS were evaluated.Paracentrotus lividusadult females were randomly divided into three groups and housed in tanks at 17 °C. In two of these tanks, the temperatures were gradually increased up to 23 and 28 °C. Celomatic fluid was collected after 3 and 7 days. The coelomocytes were morphologically typed and evaluated for their mitochondrial membrane potential (MMP), lipoperoxidation extent (LPO), and hydrogen peroxide content (H2O2). Respiratory burst was induced by treatment with phorbol 12-myristate 13-acetate (PMA). HS caused a significant change in the coelomocytes’ type distribution. MMP increased in the 23 °C-group and decreased in the 28 °C-group at both 3 and 7 days. LPO only increased in the 28 °C-group at 7 days. H2O2progressively decreased together with the temperature increase. Respiratory burst was detected in all groups, but it was higher in the 17 °C group. In conclusion, the increase in temperature above the comfort zone for this animal species affects their immune cells with possible impairment of their functions.