Science aside of the day:
Well, and T lymphocytes. “T cells” are a type of white blood cell involved in the immune response. They’re special because they undergo something called somatic recombination.
Try to remember back to high school biology. During meiosis (the formation of gametes) there’s a step where Chromosome 1 from Mom and Chromosome 1 from dad can swap chunks of DNA – that’s recombination. It’s the reason why we have so much diversity – because you’re not just getting Grandma or Grandpa’s chromosome, you’re getting a mix of both.
Usually this only happens when making sperm or eggs, but there’s one time it occurs in non-gamete (somatic) cells – in the production of T cells. A protein called a T cell receptor recognizes antigens (foreign particles) from viruses, bacteria, parasites, and even tumor cells. But think of it – if there was just one gene coding for a T cell receptor, we’d only be able to recognize one type of antigen. That’s no good – we need to be able to recognize millions!
Thankfully evolution has the answer. The T cell receptor gene has three main segments: Variable, Diverse, and Joining. There are 65 V, 27 D, and 6 J – but the cell only needs one of each! That’s where somatic recombination comes in – it randomly deletes all but on of each segment, leaving the cell with a unique combination.
“But wait,” cry my more mathematical readers, “that only leaves 10,530 combinations! That’s not very diverse at all!” You’re right! These huge structural differences make up most of the diversity, but these genes are also hypermutable – they gain mutations WAY faster than other genes. So that contributes to the diversity even more!
So, are we ready to start calling every T cell a person because it has a unique human genome? I’m not sure if my psyche can stand all the funeral’s I’d be having every time I get sick.