Immune: A Journey Into the Mysterious System That Keeps You Alive

by Philipp Dettmer

This is it—no other cell is allowed to have an MHC class II molecule.*2 In contrast to that: Every cell of your body that has a nucleus (so not red blood cells) has MHC class I molecules. OK, why is that and how does this work? As we said before, cells are constantly breaking down their proteins so their parts can be recycled and reused. The crucial thing here is that while this recycling happens, your cells pick a random selection of protein pieces and transport them to their membranes to display them on their surfaces. The MHC class I molecule showcases these proteins to the outside world, just like a fancy display window would showcase a selection of the items the inside of a store has to offer. This way, the protein story of what is going on inside the cell can be told to the outside. To make sure the story is always up-to-date, your cells have many thousands of display windows—or many thousands of MHC class I molecules—and each one gets refreshed with a new protein, about once a day.