In people having heart surgery, a special type of immune cell called iNKT cells is found in much higher amounts in the fluid around the heart than in the blood — and it's the only immune cell type that shows up more in that heart fluid.

What we've found so far is that iNKT cells appear to be more concentrated in the fluid around the heart than in the blood of people undergoing cardiac surgery [1]. The evidence we’ve reviewed leans toward this being the case, and so far, no studies have contradicted it. Our analysis of the available research shows that in patients having heart surgery, a type of immune cell called invariant natural killer T (iNKT) cells is present at higher levels in pericardial fluid—the fluid surrounding the heart—compared to blood [1]. This difference is notable because, among all immune cells, iNKT cells are the only type reported to show this pattern of being more abundant in that specific fluid [1]. The data supporting this comes from one claim backed by 27.0 studies, with no studies indicating the opposite. We don’t yet know why this concentration difference exists, and the evidence doesn’t tell us whether it plays a direct role in heart health or surgical outcomes. But the consistent support across studies suggests this pattern is worth further exploration. Since our understanding is based on limited claims so far, future research could refine or expand on what we’re seeing. For now, what this means is that in people undergoing heart surgery, iNKT cells are more densely located around the heart than in circulating blood—a clue that these cells might be positioned to play a specific role in the heart’s local immune environment.