Which molecules of the adaptive defense system provide humoral immunity by circulating freely in the blood and lymph, where they bind to extracellular antigens and inactivate them and mark them for destruction?

Adaptive immune defense system consists of lymphocytes like B-lymphocytes and T- lymphocytes. B-lymphocytes provides humoral immunity while T- lymphocytes provide cell-mediated immunity to the body.

99% of lymphocytes circulate freely in the blood and lymph. B lymphocytes differentiate into plasma B cells and B memory cells when interact with antigen presented by T helper cells.  

Then plasma cells secrete antibodies in the circulation which binds to extracellular antigens through antigen-binding site. Then the bounded antigen is recognized by receptors present on phagocytic cells. This receptor binds the Fc region of antigen bounded antibody and destroy the antigen by phagocytosis.