The Song of the Cell: An Exploration of Medicine and the New Human

by Siddhartha Mukherjee

The CD4-positive T cell sits at the crossroads of cellular immunity. To call it a “helper” cell is to call Thomas Cromwell a mid-level bureaucrat; the CD4 cell is not so much a helper as it is the master machinator of the entire immune system, the coordinator, the central nexus through which virtually all immune information flows. Its functions are diverse. Its work begins, as we read before, when it detects peptides from pathogens, loaded on class II MHC molecules, and presented by cells. Then it jump-starts the immune response, activating it, sending alarms, enabling B cell maturation, and recruiting CD8 T cells to sites of viral infection. It secretes factors that enable cross talk between the various wings of the immune response. It is the central bridge between innate immunity and adaptive immunity—between all the cells of the immune system. The collapse of CD4 cells thus cascades rapidly into a collapse of the immune system in total.