Assertion: B-cells work chiefly by secreting substances called antibodies into the body fluids.
Reason: Antibodies ambush foreign antigen circulating in the blood stream.

Both Assertion (A) and Reason (R) are correct and R is the correct explanation of A.

The primary and secondary immune responses are carried out with the help of two special types of lymphocytes present in our blood, i.e., B-lymphocytes and T-lymphocytes. The B-lymphocytes produce an army of proteins in response to pathogens into our blood to fight with them. These proteins are called antibodies. The T-cells themselves do not secrete antibodies but help B cells to produce them.