Some time ago, I made a post (actually two) about the innate immune system where I briefly explained the function of Natural Killer Cells (NKs).
Because of this, I won't go into too much detail about them. Instead, I will show you some fun gifs of what we did in the lab yesterday!
We were interested in the killing behavior of NKs, as there are two ways in which they can kill other cells.
The first step is the same, they come in contact with a potentially "bad" cell and check for certain structures on the surface. These structures either inhibit the killing reaction or encourage it.
After that, they either connect to a receptor at the target cell, which causes a cascade of reactions that lead to the cell's death, or they set free a number of granules, which contain enzymes that also lead to death. The second way is faster, but it's not 100% clear how the NK chooses which way is the appropriate one.
For our experiment, we used a "Jurkat" cell line, which was T-lymphocytes that are missing the MHC-I, a for the immune reaction important complex at the outside of the cell.
The NKs came from the "waste" that is created when cleaning donated blood. There are several steps required to separate them from everything else, but we didn't do that ourselves.
The Jurkat cells and NKs were both dyed with different fluorescent dyes, green for the Jurkat cells (which dyed the cytoplasm) and red for the NKs (here the granules collected the dye). They were then measured with a fluorescence microscope over the course of an hour, the microscope took a picture every 20 seconds.
After analyzing the results, I was able to create these gifs:
Here you can see a NK swim past a Jurkat cell, then come back to attack and kill it. Note the rapid loss of green color! It means the cell is leaking. There's also a faint red invading the cell, which suggests that granules were used in the killing process.
This NK seems to be killing two Jurkat cells at the same time. See how it pushes between them and they both start losing color? Again, they're leaking.
My last NK didn't kill one, not two but three Jurkat cells during the hour it was observed. It seems to linger a bit after killing the first one, but then rushes towards the others.
I like how well you can see the fact that NKs aren't just tiny balls the whole time, as it stretches out at the end. One tends to imagine cells as something stationary or controlled by, for example, the currents of blood. But these NKs move very much by themselves!
Monster GIF was created for me by 
