If the immune system was an army,
immunology would be Star Wars….

“THE WAR has been going on for as long as I can remember. The Aliens are constantly attacking us. Why? We haven’t a sodding clue, but they keep coming and we just have to keep on fighting. Intelligence has classified our enemy into three main types. We call the big mothers that steal from our supply lines and try to weaken us, ‘parasites’. Bacteria are the standard alien attack units but there are too many different types of the slimeballs to keep count of. The worst of the lot are the viruses, these little blighters can actually get inside our people and control them to make more viruses.

We’ve got plenty of people working in our defence force to repel these scum. Sure, some have managed to get through, but not for long. We’ve always managed to deal with them. I’m not boasting here, but we’ve got enough weaponry here to kill our own population many times over – and that’s why we have to be careful what we attack.

Our first line is the APC Infantry, made up of our finest attack units - including the macrophages. These will eat and tear apart anything in sight that looks out of place. They’ll bring along anything potentially interesting to our inspection stations, the lymph nodes and lymphoid tissues. The Helpers of the T lymphocyte division (codenamed CD4) are responsible for coordinating our response. The CD4 T helpers constantly check the macrophages and other APCs to see what they’ve been eating. If a T helper sees something foreign dribbling down the face of the APC, then we’ve got a potential security breach on our hands.

The CD4 T helper sends a chemical signal via lymphokines to the appropriate boffin at the B Lymphocyte division in the lymph node telling it start producing our own chemical weapons: the antibodies. The B lymphocytes change into plasma cell units which are essentially factories to produce the massive amounts of antibodies we need. B Lymphocyte Intelligence has B memory cell specialists who keep the alien on file in case they attack again. The vermin may have slipped through once, but if they show their ugly faces again, they’re in for a big surprise.

The antibodies seek out and attach specifically to the aliens in question and light them up like a late night house party down a quiet street. Our macrophage units are issued with special receptors for the antibodies, so they’ll spot them easily, move in, and finish them off. 

The killer arm of T Lymphocyte division is codenamed CD8. The official name for our T killers is Tc (c for cytotoxic). The CD8 killers specialise in eliminating any of our own people who act abnormally, particularly those who are controlled by a virus. Harsh, I know, but this is war. We have no time here for treatment and rehabilitation. The CD4 helpers are able to signal the CD8 killers to mature into their fully activated form. Once activated, the CD8 killer will seek out and destroy anyone harbouring a virus. They can home in on chemical signals given off by a compromised individual, and other chemicals from the CD4 T helpers help them in accomplishing their duty. The T helpers also call in our macrophages, and administer the berserker agent. This makes them as hungry as Scott of the Antarctic after he‘d finished off the last of his expedition as lunch the week before. The macrophages help the T killers in their mission of destruction and then clean up the mess afterwards.

One of the ways our CD8 killers work is to induce suicide –“assisted suicide”, if you will. CD8 killers have an access code that gives our compromised people permission to lay down their lives for the good of the many. We call this process ‘apoptosis’. The CD8 killers don’t attack the alien itself because we can’t be sure what defence system they might have to resist our weapons.”

					From the Memoirs of General Intro Duckshun

Note: Some details have been oversimplified - but should give a sort of broad picture of things. Well you know what I mean....

Quick summary of cells involved:
Antigen presenting cells (APCs) – these cells chew up stuff and present the antigen via MHC class II receptors on their cell surface
Most cells in the body – periodically chop up proteins that are inside the cell and present them on MHC class I recceptors
CD4 T helper cells – these look at the MHC class II complex on the APCs and secrete cytokines when they see something they don’t like
•	T helper 1 (TH1) – helps activate macrophages and cytotoxic T cells (i.e. promotes cell mediated immunity) and suppresses TH2 cells
•	T helper 2 (TH2) – activates B lymphocytes cells (i.e. promotes antibody mediated immunity) and suppresses TH1 cells

CD8 T killer (Tc) cells – these check the MHC class I complexes for signs of antigen and kill the offending cells when there’s something amiss
Natural killer (NK) cells – (a.k.a. large granular lymphocytes) kill virus-infected cells and certain cancer cells but are restricted to using the MHC complexes. They have their own carbohydrate receptors somewhere else and acts rapidly without needing a build-up of cell numbers (unlike the other lymphocytes).

References:
At War Within by William R. Clark (pp. 35-55)
Lecture Notes on Immunology by Reeves and Todd, 3rd Edition (pp. 49-67)
Immunology at a glance by Playfair, 6th Edition (pp. 24-25, 46-47)
Medical Immunology for Students by Playfair and Lydyard (pp. 11, 17, 27, 31)

“(Barn) Dancing is the vertical expression of a horizontal desire (in a barn).”
- anon