The human body is probably the most studied biological system ever. You’d think that by now we’d know all there is to know about our lungs, skin, and kidneys. But time and again, scientists keep uncovering yet another facet of our own make-up. Now, researchers have discovered four new types of blood cells that we didn’t know existed.

Previously, types of blood cells have been identified based on the particular proteins they express on their surfaces, but this technique can miss difficult-to-identify or rare classes of blood cells. To refine this, the researchers have used something called single-cell genomics, which allows them to look in detail at the differences in gene expression between different cells. In doing so, they gain a much more accurate picture as to what cell types exist.

By doing this, they have been able to uncover four new classes of white blood cells. A crucial part of the immune system, white blood cells are subdivided into different types that play different roles in the fight against infection. This latest study, published in Science, has found two new types of dendritic cells and two new types of monocyte cells.

Dendritic cells play a crucial role in linking the innate immune system, which involves the defences such as skin and membranes that help fight infection, and the adaptive immune system, in which specialized cells process and eliminate pathogens, as well as retain a memory should they face the same infection again. The role of the dendritic cells is to take up fragments of the antigen infecting the body and then present parts of it on their surface to “educate” T-cells, which then hunt down and kill the pathogen.

The monocyte cells provide a different role within the immune system. They are the largest type of white blood cell, and can go on to form what are known as macrophages, which go to the site of infection and gobble up and digest the pathogens, removing them from the body.

“In this study, scientists have used cutting-edge technologies to find that there are many more types of cell than we originally thought,” explains Divya Shah, from Wellcome’s Infection and Immunobiology team, who helped fund the study. “The next step is to find out what each of these cell types do in our immune system, both when we’re healthy and during disease.”