The Nerdiest Thing Ever. Or the Immune System in Five Minutes or Less

OK, so I'm warning all of you that this is very very nerdy. You might not want to go any further. But if you do, know that I experienced much joy writing this and I hope you all learn something.

Also, I'll give you the background. In our many classes, we have "learning objectives": Ten-thousand small things we need to know to pass the course. There are so many of them, our class decided to split them all up between us and enter them all in a google doc online. Some of them are as simple as "define necrosis". Others are not:i) In your own words, restate how the various components of the immune system interact when the body is presented with an antigen. This latter was my first assignment. And here, ladies and gentlemen is what happens when lack of sleep + extra coffee + a really complex topic + a sprinkling of sincere devotion to biology will get you.

I think if you ignore the terminology, you might like it too.
----------------------------------------------------------------------

First, a baddie (microbe or unwanted character of some kind) enters the scene and tries to get past the first lines of defense:
Skin, pH, sweat, tears, sebum all try to block his way.
He gets through the epithelium of the skin or GI and encounters the Innate Immune Response.

Innate Immune Response – Phagocytes (neutrophils and macrophages) see the baddie and his friends.
- Phagocytes will eat microbes and secrete cytokines, stimulating inflammation and a lymphocyte response (see below). There are also some specialized cells, Dendritic cells (more info below) and Natural Killer cells. NK cells march around the body just looking for baddies to kill.
- Complement system (proteins circulating in the body) will activate and take part in a number of activities including forming MAC – Membrane Attack Complex. MAC proteins see the baddies and attach to their membranes forming pores and ultimately lysis.
- Back to the dendritic cells – they are specialized cells that carry around chewed up antigens (proteins) that come from destroyed baddies. These dendritic cells are also known as Antigen Presenting Cells (APC’s) because, well, because they are cells that present antigens. They travel to the lymph nodes to present their chewed-up bits of bad guys to naïve T-cells. This stimulates proliferation and differentiation of T-cells.
- T-cells – now the T-cells become active T-cells. There are 2 kinds of active T-cells – T helper cells (CD4+) and T cytotoxic cells (CD8+). T cytotoxic cells kill (esp. virally) infected cells and tumors. T helper cells activate macrophages to become even more efficient killers and stimulate B-cells to enter clonal expansion and produce antibodies (more on this below)

Adaptive Immune Response –
- T-cells complex (get together) with B-cells and tell them (through the use of cytokine (chemical) messengers) to make antibodies.
- B-cells go into clonal expansion – this means they turn into plasma cells and make lots and lots and lots and lots and lots of antibodies against the antigen (remember the baddie?)
- Antibodies do all kinds of wonderful things and there are several different ones. Briefly, they neutralize baddies, opsonize (coat them so that they seem more delicious to macrophages), activate complement, protect against parasites, and protect newborns.

The body then hopefully returns to its basal state – the state of normalcy it held before. One difference is that it can now have memory cells. If the body encounters this particular kind of baddie again, it can skip over a bunch of steps and go straight into clonal expansion because it will now have memory cells.

And that is it in a nutshell.