Origins

Reptiles look old school, and they have old school B cells that retain an ancient ability our B cells have lost, says a new study published today. Our B cells cannot engulf invading bacteria, but a turtle's can. The results help narrow down when the immune system's antibody factories stopped dining out.

The mammalian immune system divvies up the pathogen-fighting duties. Macrophages and similar cells perform phagocytosis, eating and destroying bacteria and other invaders. Instead of tangling with pathogens, B cells counterattack by pumping out antibodies that home in on intruders. According to immunologists, mammalian B cells aren't capable of phagocytosis.

Researchers infer that phagocytosis came first—single-celled organisms such as amoebas use the maneuver to capture food—and that B cells evolved from phagocytic cells (see the Origins essay on the evolution of the immune system). A 2006 study bolstered that hypothesis, showing that cells from fish and frogs have both abilities.

The question is when in vertebrate evolution some B cells lost their appetite for pathogens. Nobody had run a taste test on reptile cells, so a team from Illinois State University in Normal offered fluorescent beads to B cells from red-eared sliders, a kind of turtle (above). Some of the cells snarfed up the beads, indicating that they were capable of phagocytosis, report Laura Zimmerman and colleagues in Biology Letters.

These dining habits suggest that the B cells of mammals didn’t give up phagocytosis until after the group parted from reptiles. The researchers propose that B cells' double duty reinforces immunity in reptiles, whose antibodies aren't as potent or produced as quickly as those of mammals. To sharpen their picture of B cell evolution, researchers now need to determine whether bird B cells also abstain from phagocytosis.

Photo credit: Trisha M. Sears