Tomorrow marks our fourth medical exam this semester, this time in immunology. Naturally, this delicately balanced, devilishly complex field would be the one course in which we don’t have midterms; everything rests on the final examination. It’s infuriating, but logical when you consider that medical education now takes even fewer years than in the pre-genomic, pre-antibiotic, pre-molecular biological era. I think this is why our oath includes that clause about staying current in our field.

At any rate, I’m only griping because immunology is one of the coolest fields around, and is constantly changing. 20 lectures just can’t do it justice. Our professors do try to make the condensed material accessible, though, with quite entertaining results. So, for today’s study break, I thought I might share some of the explanations:

Concerning the differences between MHC Class I (how your cells present self molecules to the immune system) and MHC Class II (how special cells called Antigen Presenting Cells – or APCs – present extracellular molecules to the immune system):

  • It all boils down to Lindsay Lohan’s arrest record. Say you’re walking by a kindergarten, minding you own business,when you see a plastic baggie full of white powder sitting on the pavement. Now, if you’re incredibly naive, you pick the bag up and put it in your pocket, thinking that you’ll just throw it in the trash can down the street, but you forget. Congratulations! You are now an infected cell! Next day, a cop stops you for walking and chewing gum at the same time, and he makes you turn out your pockets. In there with your keys and your wallet and cell phone is the little white baggie. You say, “Gee, officer, that white bag isn’t mine, I just picked it up somewhere…” Guess how far that’s going to get you.
    Now, if you’d been an APC, you would have seen that bag on the pavement and immediately thought to yourself, Hmmm. I bet Lindsay Lohan’s been here. Don’t want any of these li’l tykes picking this up, so I’d better turn this in and get the cops to canvas the neighborhood. You double bag the suspicious evidence and flash your undercover MHC II license back at the station, so next time Lindsay shows her face, she’s toast. Case closed.
  • Of course, there’s more to MHC I and and MHC II than just where they get the peptides that they present. There are also different rules for how big the peptides can be. Think of it as New York versus San Francisco. MHC class II presents foreign peptides, and is a way of asking for help controlling a problem, so it’s pretty lenient. Pack a bunch a’ suspicious characters into a trolley car, let ’em dangle out the windows a bit, and then just shove ’em off towards Alcatraz. No worries. MHC I, on the other hand, tells the immune system whether you’re doing ok or have been infected by a virus, and the penalty for infection is death. Your cells are paranoid about what gets onto these things. It’s New York. It invented exact change. 8-10 amino acids or you don’t get on the bus.

Concerning immunoFAIL:

  • If the immune system worked perfectly all the time, we wouldn’t have doctors. Pathogens are pretty smart, too, especially HIV, which is able to hijack your helper-T cells and kill them to prevent activation of the killer-T cells, which could actually clear the infection. As we learned, the changes over time in this disease are kind of like replacing Eli Manning with Mark Sanchez.
  • HIV is a retrovirus, meaning that when it enters a cell, it back-transcribes itself into your very own genome. It’s pretty savvy, and has worked out that the best strategy is to build up a new army before busting out and invading more cells. So, it has to sequester a lot of its players inside the nucleus until it’s time for them to enter the cytoplasm. Think of the cytoplasm as an exclusive new club in the Seattle Space Needle. It has a fussy bouncer named Rev. Now, Bill Gates reaaaallly wants to get into this club, but as a young man his CRS nerd factor is just too high. He gets bounced like an olympic pingpong. However, a little later in life, he acquires RRE, which in this universe stands for gigantic piles of cash. This RRE bling is so thoroughly attractive that it outweighs the nerd factor, and the once disdainful Rev gloms onto Bill like a limpet and escorts him into the club for life.

And that, my friends, is how I will forever remember immunology.

P.S.

There are some more fun anecdotes, but my study break is up, so they’ll have to wait for later.

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