Health providers are trying to stay away from immunosuppressants unless it is absolutely necessary because these will totally impact any positive response to the virus, as well as a negative response such as cytokine storm. In addition, remember that people who are already immunosuppressed are particularly susceptible to the virus. Immunosuppressants were tried in the SARS epidemic and this led to an increase in viral load and delayed recovery and may increase damage. Studies have shown that use of immunosuppressants does not increase survival.
Donated plasma is scrupulously examined for any trace of pathogens, is pretty safe.
It's simple; just spin the blood down. Blood is drawn into a tube that contains an anticoagulant so the blood does not clot. Cells in the blood go down to the bottom of the tube during centrifugation because the cells are heavier leaving plasma above the cells. The plasma is just removed leaving the cells behind.
No, antibodies given to someone are just proteins. Proteins will naturally decay. Antibodies cannot stimulated the production of more antibodies.
Not if it kills you first. Cyanide is extremely short acting and is lethal. You can recover from a sublethal dose of cyanide but it can affect the brain and cause psychiatric issues. Look up the arsenic eaters if you want to find an interesting view of nonlethal ingestion of a poison.
Minutes. For example, the pregnancy test is based on producing a particular protein called hCG. The rapid test for SARS-CoV-2 works within minutes using the same principle as a pregancy test. These are known as antigen tests. If the antigen test is positive, then there is pretty good chance of infection. However, a negative test does not mean that someone has not been infected. There is a high percentage of false negatives with these tests.