Which cells secrete the antibodies?

Plasma cells secrete antibodies.

Plasma cells are a type of white blood cell that are produced by the differentiation of B cells. B cells are activated when they bind to an antigen, which is a foreign substance that the body recognizes as harmful. Once activated, B cells proliferate and differentiate into plasma cells. Plasma cells then secrete antibodies, which are proteins that bind to the antigen and help to neutralize it.