Not Second Class: The First Class II MHC Crystal Structure

Classical MHC proteins come in two flavors: class I and class II. In some ways, it can seem that class I MHC molecules have lots of advantages over class II: class I molecules are expressed on almost all cells instead of just a subset of immune cells and are recognized by exciting and dangerous "killer" T cells, whereas class II molecules are targeted by seemingly more benign "helper" T cells. However, class II MHC molecules have their own story to tell, and the first crystal structure of a class II MHC protein (1), solved in 1993, was no exception.