Author's Note: Since NK cell is back, I thought it'd be appropriate to share this! I wrote it a long time ago for Cells at Work Week before I was unable to participate in full. But anyway, it helps me remember a particular feature of the immune response to CMV.
"Sir?" A formerly naïve T cell pointed in the direction of a lone figure, perched on a bridge over a lymph channel. "What kind of cell is that?"
At his subordinate's low question, a certain killer T cell stopped in his tracks, and his blond eyebrows drew together as he scowled. "Ah. Listen up! Here's what you need to know about that cell…"
The first time he saw her, it was during a particularly nasty battle. One neighborhood of cells had been hit by a virus and the killer T recruits were sent in to control the situation. Amidst the haze of battle, he was shoved into an alley, scraping the walls. He felt like the force cracked his cytoskeleton and he groaned.
"Whew. Looks like you're having a rough time."
The unfamiliar voice made him look up to see a strange cell, crouched in front of him. A sleeveless black shirt. Sturdy arms that could only be designed for fighting. A machete at her belt. He had glimpsed cells like her before, but that was in his very early days in the bone marrow.
"Yeah. You're a lymphoid cell, aren't you? Can you help out?"
"Depends." She shrugged.
"Depends? We're in a crisis here and we need every able fighter. If you care about the well-being of this body, you'll do your job!"
She didn't answer at first, her gaze running over him. "A killer T. Figures."
"W-what's that supposed to mean? And what are you?"
"Why don't you ask the rest of your platoon? I'm sure you can find them by all the noise they're making."
"Hey! Look, we can talk later, but we need to survive first!" He gestured to the open streets, just as an infected wandered past. The cell backtracked, protein-studded cap bobbing.
The other lymphoid cell stood, rolled her shoulders, and moved to intercept. Finally, she was listening. She grabbed the front of the infected cell, lifted her weapon, and…stopped. "Hm. Not my problem this time. It's all yours!" She let go, and the infected cell lunged straight for the killer T.
He screamed.
From overhead, a blur of white frills landed in front of him. "Oh my! You're a naughty one, aren't you?" With a sugary giggle and a spin of the macrophage's axe, the infected cell staggered backwards.
It was enough for the killer T cell to step up and follow through. The rage at the strange cell surged into his fist, and the infected cell went down. "And stay there, you bastard! Damn it! Did you see that, you-huh?"
The other lymphoid cell was nowhere to be seen.
"If you're looking for the NK cell, she slipped away." The macrophage offered.
"NK cell…tch!"
"What the hell is wrong with you?!" Sure, it was the middle of patrol and the surrounding cells flashed him apprehensive looks, but he wasn't about to let this rogue NK cell get away. She was already halfway down a staircase, winding around a residential building.
She scoffed. "Think of a better way to get a cell's attention. I don't have time for this." She leapt over the rail, disappearing over the side.
He wouldn't be able to catch her at this rate! Gritting his teeth, he jumped as well. Too late, he realized he made an error in judgment. From here, he would land on a crate of glycogen and he was suspended for an instant, before he caught the ledge. Nanometer by nanometer, he shakily clawed his way back up and glared as he sprinted down the stairs.
The NK cell hadn't run off after all, taking a point next to the front door, and she snickered. As she doubled over, clutching her stomach, she erupted into full-throated laughter. It might have sounded nice, if his fingers weren't throbbing. "Oh, man!" She gasped and wiped a tear from her eye.
"You…NK cell. I've heard of your kind. Not as powerful as the macrophages, not first to defend like neutrophils. You just lurk at the edge of battle, whenever and wherever."
"Well, what about you killer T cells?" She retorted. "With all your shouting and marching, any bacterium could hear you coming from a capillary bed away."
"It's necessary to be vigilant! And what job do you have, that you can ignore an infected cell? Can't even take one down, when it's right in your hands?"
Her eyes narrowed. "It wasn't the right cell."
"Oh, sure, you can say that. Before that single cell could have infected another, and then another." He rested his fist against the wall and leaned in, deathly serious. "Before we lose."
"How scary." She reached over and pinched his cheek.
"Ow!"
"Can. You. Keep. It. Down." A voice interrupted. One of the residents had exited the building, standing on the front steps with a blank expression. Strangely enough, he wore a smooth hat with a wide brim, like a red blood cell. However, this cell wasn't enucleated and didn't wear the iconic uniform of a red blood cell. He did look a little malformed, as if he had stuffed blocks underneath his shirt.
"Sorry." The killer T apologized but the resident cell didn't move, seemingly frozen in place. "Are you alright?"
There was no answer.
The NK reached over to the strange cell's shoulder and clicked her tongue. "I see. We have a problem here."
"Huh? He might be a little weird, but I don't see a problem."
"Of course, you don't. You killer T cells only act if your targets have the right look about them. Lucky for you," She grinned and her fingers locked around her machete, raising it above her head. "This is my job. Covering your blind spot!" She swung down and as the cell screeched, part of the hat tore off, revealing an array of teeth. A virus.
"Your job?" The killer T was still reeling. "This is my job! Killing virally infected cells!" He stepped up and attempted a flurry of punches, but weirdly, the injured cell dodged each blow. He just couldn't connect.
Meanwhile, the NK acted like it was a buffet, already cleaning out the first floor of infected. Before heading to the next, she smirked at him. "Shouldn't a killer T be able to take down one cell, when it's right in your hands?"
"You…!"
As for the rest of that encounter, he wanted to forget it all. The macrophages came, and then, more white blood cells. But not a single killer T could terminate an infected cell. From the memory cells hovering in the aftermath, it had been something called a 'cytomegalovirus'. Whatever it was, it seemed to evade killer T attacks and the NK cells wouldn't let them forget it.
"I understand now." The formerly naïve T cell nodded. "An NK cell is our ally."
He wasn't sure how to reply to that, but the NK cell called out. "Tell him about the cancer cell too!"
"Were you listening the entire time?!" The killer T seethed.
"It's a free body. And even your whispers are loud, that's just like a killer T!"
"Lurking around, that's just like an NK!"
"Thank you for your hard work!" The formerly naïve T cell brought his hand to his hat, in a proper show of respect. "We will look for you in the next fight against the 'cytomegalovirus'."
"Salutes are for T cells." She gave a casual wave. "See you around!" And then, she strode off.
"Wasn't that right, sir? We're all working hard to protect this body, in our own ways."
The killer T's eyelid twitched. "…yes, but put your hand down. Now, come on. Let's continue on our patrol."
"Yes, sir!"
Some of the biology behind this: all nucleated cells have a protein called MHC I, which presents internally processed antigens to killer T cells. But cytomegalovirus can downregulate MHC I. No sign of viral infection, no immune response from the killer T cells, right? However, NK cells recognize the lack of MHC I, so they're able to fight back.
