Antibodies are formed by randomly shuffling the “highly variable” regions of the antibody DNA to make about every conceivable combination. This makes a “lock” that can fit around a very specific “key”. When in utero those antibodies circulate and any that recognize their specific “key” are destroyed because they recognized “self”. The rest are on alert looking for anything that might fit so it can trigger an immune response and massive proliferation of that particular antibody.
By isolating this “spike” that they wish to raise an immune response to, infecting a rabbit with it, then harvesting the resulting antibodies that can then be linked to medicine or may have a medicinal effect just by binding to and blocking the “spike” that the virus needs to enter the cell.
Antibody based medicines have amazing potential.