Complex sentence: Plasma cells can only produce a single kind of antibody in a single class of immunoglobulin, in other words every B cell is specific to a single antigen, but each cell can produce several thousand matching antibodies per second.
Split into simpler sentences: Plasma cells can only produce a single kind of antibody in a single class of immunoglobulin. In other words, every B cell is specific to a single antigen, but each cell can produce several thousand matching antibodies per second.