Adaptive Immunity: The ABC’s of APC’s, MHC’s, B’s, and T’s

Brain BREAK! presents: Adaptive Immunity.

Also known as “learned,” “acquired,” or “specific” immunity, B and T lymphocytes start out from stem cells in bones. From there, the family tree only grows more diverse! B-cells perform jobs that fall under the humoral response. Meanwhile, T-cells take an extra trip to the thymus (hence, T) gland to grow up into mature T-cells and control the cell-mediated response.

T-cell types include types like natural killer T’s, as well as our two unlikely heroes, CD-4 and CD-8 T-cells.

Setting the scene…

The CD-8 says, “I check for traitors.” Indeed, Major Histocompatibility Complex I (MHC-I) is a little grabber on her surface that she uses to analyze passing cells. Other native human body cells have MHC-I grabbers to display information to those CD-8s. It’s like a secret handshake. If the handshake doesn’t match up, the CD-8 knows what she has to do. (Spoiler: it’s tragic.)

I tell myself that MHC-I is the “I” complex, used for my cells. It’s I, I, I and me, me, me.

MHC-II, meanwhile, is a little dongle on antigen-presenting cells (APC’s), also known as dendritic cells. APC’s are just that. They gobble up passing microbes and then show off the half-digested antigen bits on MHC-II. Kind of gross, really. Chew with your mouth closed, APC. But then CD-4 T-cells have MHC-II receptors to inspect these antigens on display. “I check for bad guys,” the CD-4 cell says.

MHC-I multiplied by CD-8 = 8

MHC-II multiplied by CD-4 = 8

This is how I remember which MHC goes to which T-cell. Both multiplications make 8. Congratulations! You’re using algebra in biology.

A little B cell smiles simply, saying, “I am so naive!” It has its own receptors for reading. There are different types of Immunoglobulins, like IgA, IgD, IgE, found in different parts of the body. (IgG cells respond to a second, learned, exposure and make up the most B-cells in your body.) But today we’ll be seeing IgM B-cells.

With all of that established…

INVASION!

An APC, munching on some bits of virus, holds up a little blob of glycoprotein. They eye it, muttering, “Wait, what’s this?”. While presenting it on one of the APC’s many “arms,” a passing CD-4 cell identifies the antigen as a baddie’s and freaks out. He fear-farts cytokines, which are a chemical call to action, and then splits himself and his friends into Memory T-cells, Helper 1’s, and Helper 2’s.

We cut now to CD-8. Without an APC, she’s discovered the invasion elsewhere. This T-cell has her hands full splitting into Memory and Cytotoxic T’s. You see, her buddy Steve, an epithelial cell, isn’t looking so good. She checks out his MHC-I and finds some stranger’s antigen there – it’s too late to help him. They’ve gotten in. She has to shoot Steve down with granzymes and perforins because he’s been compromised by the enemy. “I’m sorry!” she cries, as her friend is sacrificed for the Great Cause. The Cytotoxic T’s are having this epic zombie fight while we pan back to the Helper T’s, who are running around, yelling.

The Helper T’s are like messengers reporting the news from the front lines, but the B-cells and other white blood cells (and the poor Cytotoxic) do the actual work. Yeah, I said it. Anyway, the Helper 1 T’s signal macrophages (those giant WBC’s) to get busy eating baddies. This pleases macrophages, who lick their lips and announce, “Dinner time!”

The Helper 2 T’s send interleukins (lymphocyte-to-lymphocyte wartime code) to our once-naive B-cell. The antigen is shown to the B-cell, and the air is thick with danger! Stepping up to the occasion, they become Memory B and Plasma B (IgM) cells. With all of this enemy intelligence from the Helper 2 T’s, they mass-produce a chemical weapon that will perfectly trap these specific antigens in a sticky web. They shoot out the “Y” assemblies, shouting, “Have some antibodies!” The antibodies scatter across the battlefield like caltrops. (Go ahead, look up caltrops. I’ll wait.)

And overlooking this humoral response is another macrophage. He sees this buffet of viruses, antigens, and antibodies now all clumped together, remarking, “Those B-cells really know how to complement a meal!”

So: let’s recap.

CD-8 T’s check out your cells and determine if they’ve been invaded.

APC’s eat things and display those antigens.

CD-4 T’s check out antigens presented by APC’s to sound an alarm.

Helper T-cell types 1 and 2 direct other cells to work.

B-cells are activated by Helper 2 T’s and make antibodies specific to the enemy.

Macrophages devour!

And poor Steve, the epithelial cell, will be remembered fondly by the Memory cells both B and T, who now know the secrets of how to defeat the horde of the One Glycoprotein – and record this epic in the annals of time should the enemy ever return. And it will. Someday… it will.

Such drama!! -CN^x