T cells find infections by what cells display
- The idea
- Predict
- See it
- Why
- Check
The idea
“A T cell spots an infection by sensing the virus itself, free or inside a cell.”
It's easy to picture a T cell sniffing out the virus itself. It only sees what a cell displays on its surface.
The short version
The mistake: A T cell spots an infection by sensing the virus itself, free or inside a cell.
What's actually true: A T cell recognizes a piece of the pathogen that a cell displays on MHC proteins, and it has to touch that cell to read it.
- Most body cells display short pieces of their own proteins on surface proteins called MHC. In an infected cell, some pieces are viral.
- A T cell's receptor binds a piece only when MHC holds it. It can't bind a whole virus or reach inside a cell.
- So the T cell has to touch that cell. This is contact-dependent signaling. Antibodies are what bind free viruses.
On the exam: A T cell never sees the virus itself. It reads the pieces a cell displays on MHC.