Okay. Here’s my not-well-informed understanding of it.
You have two main parts to your immune system: the innate and the adaptive immune system.
The innate immune system is your first line of defence. It involves cells that look for things, such as viruses and cancer cells, that aren’t supposed to be there. They send signals, called cytokines, which cause inflammation and attract other immune cells to the area. The inflammation causes tissue damage, fever, headache, sore throat, muscle pain - general immune symptoms.
As the infection progresses, your adaptive immune system kicks in and starts generating antibodies that are specifically targeted to the infection. These stick around in various forms to fight new infections in the future.
My understanding is that why Covid affects people worse as they get older is because your innate immune system becomes less effective as you age. This makes sense - it uses a lot of energy and is pathological when overactive and, as an older person, you’ve encountered most of the pathogens you’re likely to meet before - thus, antibodies.
In severe Covid, the innate immune system kicks in too late, once the infection is already well established. Because you have systemic infection by this time, the innate system goes crazy and kicks into cytokine storm, which kills people.
We know that a strong innate immune response isn’t what kills people in Covid because younger people have a stronger innate response. It was an overactive innate immune response that killed people in the 1918 flu pandemic - and this preferentially killed younger people, not the old.
If you have a very very strong innate immune response, you will never reach the point of being hospitalised with Covid because your innate immune system will have fought off the virus. I have a pathologically strong innate immune response, which means I can be bedridden with the common cold, but I rarely develop a runny nose. Instead, I have a bizarre range of symptoms with even minor colds, which include headaches, sore throat, blue hands (!), chills, low fevers, nausea, insomnia, twitching/restless legs, rashes, fatigue, brain fog, ‘a sense of impending doom’… and so on.
Reading the accounts of some kinds of Long COVID, it appears they are due to the innate immune system never shutting down - either because the virus is still there or because it didn’t shut off. This would explain the mild initial infections with lingering symptoms.
Again, I don’t really understand this stuff, but - from touring medical practitioners with my condition - neither does anyone else…