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

by Siddhartha Mukherjee

The internal peptides, presented by class I MHCs, as Zinkernagel and Doherty had found, are detected by a set of T cells called CD8 killer T cells. The CD8 cells, you’ll remember, kill the infected cell, purging the virus in the process. In contrast, a majority of peptides derived from pathogens outside the cell (and a few from the cell’s interior that end up in the lysosome) are presented by class II MHCs. These are detected by a second class of T cells, called CD4 T cells. The CD4 T cell isn’t a killer (again, there’s a logic to this. The virus is already dead and minced to pieces; why kill the cell that’s alerting the T cell to a dead virus?). Rather, this T cell is an orchestrator. Having sensed the MHC II peptide complex, the CD4 cell begins to coordinate an immune response. It incites B cells to start synthesizing antibodies. It secretes substances that amplify the macrophage’s capacity to phagocytose. It causes an upsurge of local blood flow and summons yet other immune cells, including B cells, to challenge the infection. In the absence of the CD4 cell, the transition between innate immunity and adaptive immunity—i.e., between the detection of a pathogen and antibody production by B cells—would fall apart. For all these properties, and especially for supporting the B cell antibody response, this type of cell is called the “helper” T cell. Its job is to bridge the innate and adaptive immune system—macrophages and monocytes on one end, and B and T cells on the other.