Models (14–16): Immune system
The immune system is often taught within the metaphor of a battle. Yet immunity is really about identity: a homeostatic system that maintains a sense of a cohesive whole. If pathogens enter, the organism’s identity is compromised, and it organises itself to bring the body back to its autopoetic self. Likewise, pathogens aren’t attacking us; our body is just an environment, a particularly good one, to live in. They, too, are just maintaining their autopoiesis. As I argue in Biology Made Real, identity is central to meaning making.
Microorganisms and viruses
As I’ll be discussing forms of pathogens, the best place to begin would be distinguishing pathogens from other microorganisms. This allows us to revise forms of life and how their cells differ. And, then, distinguish viruses as nonliving. As I argue in Biology Made Real, meaning here comes from evolutionary history, and so I discuss the domains of life and the absence of viruses.
First line of defence and blood clotting
One of the fundamental problems in life is the tension between openness and closedness. Organisms must be open to obtain energy and matter; otherwise, they’d dehydrate, starve, and suffocate. But this brings problems: being too open would allow all sorts of things, molecules and microorganisms into the body, such that it loses its identity and autopoiesis. By losing autopoiesis, the body loses its ability to self-renew, and death ensues. Therefore, it must also be closed; it needs a robust barrier. This lesson begins here and distinguishes between skin and mucous membranes. Then we co-construct a stock and flow model to understand the mechanism of blood clotting. Learn how to teach with stock and flow diagrams in Difference Maker.
Phagocytes and innate immune response
Phagocytosis is quite an easy mechanism for students to learn and memorise, probably because they find the ideas of pursuit and consumption intuitive. In this lesson, I spend more time distinguishing between the innate and adaptive responses of our immunity. The adaptive response of lymphocytes is only found in vertebrates; invertebrates get by just with an innate response. Why is that? The meaning, as I argue in Biology Made Real, is found in the evolution and the organism’s way of life. In this model, we discuss lifespan and generation time as the principal cause. Longer-lived vertebrates will need to fend off many pathogens during their life. During this lifetime, pathogens around them will be evolving, and they’ll need a system to match this adaptation. Insects, on the other hand, may not live long and have such large populations that they can rely on adaptation at the level of the population.
After teaching the mechanism of phagocytosis, we co-construct the small stock and flow model, which I’ll expand on in the next lesson.
Lymphocytes and adaptive immune response
This is the most complicated stock and flow model of the course so far. For this reason, I included the pathogen stock and phagocytosis in the previous lesson. Here, then, I just begin by asking them to recreate it from memory. What many teachers get wrong is trying to include stock and flow models in their classes sporadically. In my long experience developing this way of teaching (see Difference Maker), this fails, especially for the weaker students. Stock and flow modelling must be a way of doing biology (see Teaching Meaning). If it is, the complexity of this model is manageable. As always, we co-construct the model step by step.
Secondary immune response
This lesson co-constructs the typical secondary immune response graph, and then returns to the previous model to add memory cells:
Memory cells added to the model
Vaccination
This lesson is mainly spent narrating Edward Jenner’s hypothesis and the creation of the first vaccine. I have another model for this story taught to students 11–14 here.
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