Final answer:
Antibodies are produced by B cells when they differentiate into plasma cells after being activated by an antigen and receiving signaling cytokines from helper T cells.
Step-by-step explanation:
Antibodies are produced in a B cell by the differentiation of plasma cells. After a B cell is activated by an antigen, it digests the antigen and presents it on its surface with MHC II to a helper T cell. In response, the helper T cell releases cytokines that signal the B cell to differentiate into a plasma cell, which then produces and secretes antibodies. These antibodies play a crucial role in the body's humoral immunity by binding to pathogens and neutralizing them. Memory B cells are also produced during this process, which helps the immune system respond more rapidly and effectively upon subsequent exposures to the same pathogen.