It’s virtually impossible to talk about death and disease in humans without coming across the immune system. Though discussion in the series ‘Killing your character convincingly‘ is so often about immune systems that don’t work, to understand why failure can be such a devastating factor we really need to understand what it is and know a little about how it works and why it fails.

What is the immune system?

The immune system is a network of cells, tissues, and organs that work together to protect the body.

It sound so simple, but it is a complicated subject. As the name implies, the immune system orginates in a variety of places in the body all providing various agents doing various things.

The point of this article is not to bury us all under the minutae of immunology and medical jargon, so lets not do that thing. Immunology is cell warfare and everyone loves a good scrap, so lets attack this thing sideways.


 First and foremost –  Who is the enemy?

The immune system protects the body against attacks from an array of nasties (pathogens) that infect the human body and do damage: bacterial, viral. fungal, prion (responsible for mad cow disease), algal (as in blue green algae) and parasites (worms etc)


First line of defence

The first line of defence of any city is it’s walls and sentries and the human body is no different.

Just existing in an environment leaves every body open to infiltration by unwanted foreigners. Dust, dirt, pollen, spores, bites, stings, virus’s, bacteria’s and the odd solid object, the body needs to deal with and deny access to the delicate internal system before they get a chance to become nasties and do damage.

Anatomically speaking the body’s  ‘city walls’ have a range of effective sentries. Hairs in the nose and on the eyes prevent particles of dirt and dust etc getting into places they shouldn’t be. Tear ducts wash the eyes clean. Mucus is produced in the nose, nasal passages and pharynx (the bit beyond the nasal passage and above the tubes that go to the stomach and lungs). Mucus is full of good stuff, proteins and enzymes etc, that trap inhaled unwanted foreigners and get rid of them. The mucus also supports the cilia (tiny hairs that beat back and forth) that draw the mucus up and help remove the nasties in it.

The stomach and gut produce enzymes, gastric and bile acids, chemicals. proteins and micro-organisms to protect them against anything that shouldn’t be there. And should the something be a bit more indigestible, then peristalsis (wave action) will push it out.

Second line of defence.

If a pathogen gets past the first line of defence, then body sends in the first wave of immune sentries in the form of the innate immune system. The innate immune cells are our oldest and simplest defence and like the foot soldiers, they can only do what they are inherently able to do and can’t learn and adapt. ..

The job of the innate immune system is to find the pathogen, take basic defensive steps and call for back up through cell signalling. This is done by secreting chemical messengers that bind to a specific cell and change it (man to man combat). By attaching to the surface of a cell and changing the cells around it (short range artillery). Or by trggering glands elsewhere to release hormones through the circulation (long range artillery)