Explaining your immune system using Aliens: Special Edition

Aliens (1986) is James Cameron’s action sequel to Alien (1979) where Ellen Ripley returns to exomoon LV-426 to find it infested with aliens and a newt. The Special Edition is longer which you can get on DVD and contains a scene that we’re going to need.

Your immune system is the amazing coordination of white blood cells that are fighting ‘ill-making particles’ (pathogens) in your blood and tissues right now. We are going to use the film, slightly out of order, to explain how it works. First we skip the safe, talky bit and get quickly to Ripley and the Colonial Marine Unit being attacked by aliens from all sides.

First line of defence

Your skin is the main reason you are not ill all the time. Pathogens can’t get through and are soon washed off. Assuming you wash. You do wash don’t you? Here are the team putting a barrier between themselves and the aliens by barricading themselves in the colony command centre. Good work team!

 

Second line of defence

Our marines represent white blood cells (leucocytes) — that make up a small part of your blood but all of your immune system. This, second line of defence is the non-specific immune system where white blood cells called phagocytes destroy any particle that looks foreign. In your body this is done by engulfing them, breaking them up then spitting out the bits. For our film we can show any picture of someone wildly shooting a gun.

Randomly shooting at an alien.

Third line of defence

This shooting is all very well but we are going to need a more specialist approach to combat such a specialist creature. Memory would be helpful too, in case we encounter this creature again — you know, in case there are any more sequels. Ripley is already our memory cell in this film which is principally why she is there. Her briefing to the Weyland-Yutani Corporation allowed a bigger and faster response the second time.

The specific immune system uses two different approaches: Bullet With Your Name On It, and Kill The Infected. The first is a custom, ranged weapon that shoots at free-floating pathogens; the second is mercy-killing of already-infected, human cells.

Bullet With Your Name On It

 

In the Special Edition, the marines assemble sentry guns to keep the aliens away from the command centre. These bullets represent antibodies — free-floating molecules that attach to proteins on the pathogen’s coat (antigens), stick them together making them unusable, and mark them for destruction by phagocytes. Plasma cells generate 2000 of these each second which is a rate of fire you wouldn’t want happening by mistake, so we have a clever double-check mechanism before it starts.

First a type of phagocyte — a macrophage — engulfs a pathogen, breaks it up, and displays bits of it on its outside. There isn’t a scene in Aliens where this happens. John Hurt? He sort of displays the pathogen on his outside but he’s in the wrong film.

Macrophage being inspected by a T cell.

There are many white blood cells called T cells, each covered with a different receptor. These correspond with the many types of antigens (coat proteins) a pathogen might bring with it. When the right type of T cell binds with the bits of pathogen on the outside of the macrophage it duplicates into activated T helper cells.

Meanwhile, a white blood cell called a B cell has also engulfed a pathogen and has also presented bits of it on the outside. When an activated T helper cell binds with this the double-check is complete and it releases a chemical that stimulates the B cell to duplicate into B memory cells (like Ripley) and Plasma cells — the sentry guns. Blammo.

Kill the Infected

In Aliens the marines find surviving colonists have huddled together in the atmosphere processing station so go down there to give them a foil blanket and a bit of Kendal Mint Cake. There they find ‘Mary’, alive but impregnated with alien eggs. Fitting with our metaphor, Mary whispers “Kill me” — which we’ll take, even though that may not be exactly what subsequently happens.

 

Our own cells will present bits of the invading pathogen on the outside if they are infected. A white blood cell called T killer patrols around checking outer proteins on our cells. When it finds an antigen that fits its receptors it will duplicate into an army of T killer cells, plus some T memory cells, that march off dispatching any body cell that is displaying the same antigen. Cold.

Working together, these systems keep us remarkably well. Our bodies end up better off than LV-426 colony Hadleys Hope — not that I will tell you what happens. Next time you are at home fighting a cold be sure to watch Aliens: Special Edition as this massive battle takes place!