https://www.livescience.com/newfound-prenatal-immune-cells

Although B-1 cells were first discovered in mice in the 1980s, the existence of such cells in humans has been hotly debated. A new study confirms and extends research from 2011 and provides evidence that B-1 cells emerge in early human development, within the first and second trimester. In mice, these cells produce various antibodies when activated. Some of these antibodies latch onto the mouse’s own cells and help to clear dying and dead cells from the body. Activated B-1 cells also make antibodies that act as a first line of defense against pathogens, like viruses and bacteria.