I am not an immunologist but I’ll give it my best shot.
Firstly, the real expert, Prof Judi Allen, is delivering a lecture in this video and I would advise watching it, even if a lot of it goes over your head (it did mine) because with a few careful views you can pick up the basic elements. The lecturer has a tendency to speak quickly sometimes re complex issues, but try to just stay with it and understand the basic stuff as much as you can, while accepting that some will go over your head. If you feel inspired to learn in greater detail you can learn a lot using the www and the academic literature.
So my, possibly flawed, understanding of it is as follows:
From what I can discern, Prof Allen posits that there are 2 components to the human immune system, each one operates separately, so it can be posited that there are 2 separate human immune systems.
The first, Th1/17, is like your personal army of fighters that deals with huge numbers of tiny invaders. Heavily armed with a variety of weaponry, should your body be invaded by a virus or bacteria, your body’s army rushes out in force and fights the invader.
This army is a rapid response force and it is huge. It needs to be because harmful infections threaten to overwhelm the body and possibly kill you. Thus, your body is invaded by a viral or bacterial infection, your body’s army gets tooled up and rushes out rapidly with weapons flailing in huge numbers. If all is well it overwhelms the invading bad guys and you get well.
a brief step away from the video -
Your body’s army learns from this fight about the nature of the opponent and, hopefully, develops a level of immunity should it encounter the same opponent again.
However things can go wrong with your own personal army. Your body can produce so many soldiers, in such a state of excitement that they start to attack your body and cause a hyper-inflammatory response, known as a cytokine storm. In such an instance the body’s defensive forces are fighting against the body. So you get a friendly fire situation with massive collateral damage to the point where the body’s own defence forces turn against it. This is a cytokine storm, a catastrophic immune response.
back to the video -
The second human immune system, Th2, is concerned with would healing and dealing with the damage to the body caused by helminths (parasitic worms), much bigger invaders than microbes, in much smaller numbers that create a different kind of damage.
Rather than an army, this immune system is more like your body’s internal repair force. It does not need to mobilise an overwhelming force like the army does. It is always there, labouring away, mending the damage to the body caused by injuries and by parasitic worms.
Over the millions of years that humans have evolved we have evolved alongside various helminths who have used our bodies as hosts. Some of them are relatively benign, some are extremely dangerous. Most are in the middle and do not kill us but may cause internal injuries that require wound healing over a protracted period as the human lives their life. This immune system is, compared to Th1, anti-inflammatory in its action.
link
journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1002003
Now here is the really interesting point made by Prof Allen is that the type 1 and type 2 immune systems are counter regulatory - they operate independently of each other.
Thus there is the Th1/17 immune system that is essential for fighting microbial infections, but that can, if things go wrong, cause a potentially fatal, hyper-inflammatory immune response (cytokine storm).
Then there is the Th2 system that has evolved to manage wound healing including tissue damage caused by parasitic worms and is anti-inflammatory, or has significant anti-inflammatory qualities.
away from the first video, this second video is fascinating
Associate Professor Dr Sheila Donnelly - The therapeutic benefit of a parasitic worm
More than one third of the world's population is infected with parasite worms, ranging from the relatively harmless pinworm, to pathogenic worms that cause cancer, anaemia and elephantiasis. At first sight, it may be difficult to see any possible benefit to humans from infection with these fiendish parasites. Dr Sheila Donnelly shares how parasitic worms could be used as a therapeutic treatment to cure debilitating autoimmune diseases such as multiple sclerosis, type-1 diabetes and Crohn's disease.