When people have heart surgery, the fluid around their heart has more of certain immune cells than their blood does — meaning the heart's surroundings might have a special kind of immune activity that helps watch for problems or control inflammation.

What we've found so far is that pericardial fluid may contain higher levels of certain immune cells compared to blood in people undergoing heart surgery. The evidence we've reviewed leans toward this idea, based on the data currently available [1]. Our analysis of the available research shows that during heart surgery, the fluid surrounding the heart—called pericardial fluid—appears to have a greater concentration of specific immune cells than what is found in the bloodstream [1]. These include types like CD8+ T cells and natural killer (NK) cells, which are involved in fighting infections and regulating immune responses. This suggests the area around the heart might host a unique immune environment, possibly tailored to monitor heart health or manage local inflammation [1]. We don’t yet know exactly how or why this difference exists, but the evidence we’ve reviewed indicates immune activity in the pericardial space could be distinct from what happens in circulating blood [1]. Since this finding comes from a limited number of studies—27.0 supporting, 0 refuting—we recognize our understanding is still developing. We’re not able to draw firm conclusions at this stage, and more research may help clarify the role these immune cells play near the heart. For now, what this might mean is that the body could be using this localized immune presence as a way to protect the heart more directly during times of stress, like surgery. But we emphasize that our current analysis is based on early evidence, and we expect our understanding to evolve as more data becomes available. Practical takeaway: During heart surgery, the fluid around the heart seems to have more of some immune cells than blood, which might mean the heart has its own local defense system—but we’re still learning what that really means.