by Sophie Mendell
Imagine you’ve just moved to a new high school with about 500 students. On your first day, you’re given the task of meeting every student individually. Assuming the average handshake lasts about 2.5 seconds, you’ll spend over 20 minutes just shaking hands with your new classmates. If you manage to speed through classrooms, avoid small talk, and walk quickly, this whole meet and greet process could be done in just under an hour. However, the fine details of names and classroom numbers will likely be lost. Now, imagine trying to do this exact same process, but for 5,000 students per hour, while also trying to find one specific person by recognizing their handshake alone. The cells of our immune system engage in this process every day, just to keep us healthy and alive!
One of the “hands” our cells use to facilitate this handshake is called the major histocompatibility complex (MHC) molecule. These molecules provide half of the molecular handshake needed to share information between two cells. There are two types of MHC molecules – class 1 and class 2 (see image below). MHC class 1 molecules are expressed by nearly all cells in the body, allowing our immune system to periodically check in with these cells and ensure normal functioning. MHC class 2 molecules are expressed only by certain cells within the immune system, allowing for communication regarding potential ongoing illnesses or infections. Think of MHC class 1 as a standard handshake, whereas MHC class 2 is a secret handshake only our immune system is in-the-know about! Together, both MHC class 1 and 2 molecules allow for the phenomenon of immune surveillance. This is the process by which our immune system is constantly checking for and reacting to any funny business it detects. Importantly, immune surveillance protects our body from outside threats, like viruses and bacteria, and from internal threats, like cancer.
Now in the one hour during which you’re meeting all your new peers, you’ll experience a variety of different handshakes. Some will be firm, some long, some even clammy! Like each student’s personal handshake, cells contain MHC molecules that provide slightly different handshakes to other cells. The difference between MHCs is determined by a peptide, or protein fragment, that is located on the MHC complex. Each MHC has a slightly different peptide; these peptides are determined by the cell type, its health status, and many other variables. The individuality of the peptide allows for each MHC to go from having just a handshake to a personalized handshake.
Our immune system is an incredibly complex network designed to protect our body. To accomplish this, our immune cells use a variety of handshakes to check in on each other and relay messages vital to our bodily defense. They can identify cells of interest with remarkable speed and accuracy using these fine-tuned molecular handshakes. Lucky for us, our immune system does this automatically and without us ever having to shake a hand ourselves!
