This doesn't seem to be written that clearly, at least to an average layman.
Covid-19 is puzzling. Some people get no symptoms at all, while others get severely ill and may die. The research just published shows that some of the severe cases were associated with a deficiency in proteins called interferons.
Immunity depends on B cells that produce antibodies, T cells that can destroy infected cells, macrophages that chew up debris, and other cells. These cells need to communicate with each other to produce an effective immune response, and this communication is based on chemical signaling molecules, called cytokines. Interferon belongs to this group, and is a chemical that interferes with viral replication.
Researchers found that some people with severe covid-19 had very low interferon levels. Two different causes were identified:
Some people produced an antibody which destroyed their own interferon. It sometimes happens that we make an antibody to recognize a particular infectious agent, and it incidentally recognizes (cross-reacts with) a part of our own body. This can lead to auto-immune disease, where the immune system attacks part of a person's own body. (Multiple sclerosis is an autoimmune disease.) It's not clear if the covid patients had these rogue antibodies beforehand, or if they developed early in the infection, but at the time they were severely ill, they had very low interferon and so couldn't fight the infection.
The second group of people had a genetic mutation that decreased their ability to produce interferon in the first place, so the end result was the same.
Some implications of these studies:
1. Interferon may be a helpful drug to treat some severely ill patients.
2. A test might be developed to determine if someone has this particular genetic mutation. For example, relatives of someone who was severely ill might want to know if they are similarly at elevated risk of a severe response to covid.