What’s proposed is there are two immune systems, an infectious immune system to prevent the host getting infected, and a transmission immune system to prevent the infected host passing infection on to new hosts. It’s really an idea, a way of better explaining what we observe, especially in humans, and especially for SARS-CoV-2 transmission.
While the internal aspect infection immunity has been extensively studied (think killer t-cells, memory B-cells, antibodies etc), the idea of transmission immunity is less well understood.
Why would a transmission immune system exist for us humans?
Well our survival is strongly linked to our family, our social group, our friends. That we care about them is indicative of this. More specifically, we exist to replicate our genes, there would be an overlap in our genes within our family unit. But these people would also be the ones we spend most of our time with, so when we were infectious these would be the ones we would be most likely to spread diseases to. Our transmission immune system would exist to stop this.
We can’t easily quantify how strongly this effect would be because so many of the influencing variables are unknown (eg family size, survival benefit of friends, cost of helping sick family members etc), but the argument for the transmission human immune system looks very solid. That creatures exist preferentially in herds, shoals, flocks, or in humans, tribes, indicates a survival benefit to the individual in being part of a group, and that therefore the individual wouldn’t want other individuals in the group harmed by disease.
For it to exist it would have to do things that the infectious immune system could not, it’s observed behaviour would be recognisably different from what we would otherwise expect if the goal of the immune system was only to prevent the host becoming infected.
Why does it matter? As a new paradigm it has the potential to explain a wide array of disease-related phenomena which is what we’ll be looking at here…