Which statement about antibodies is correct?

• A. They are produced by plasma cells.
• B. They secrete antigens.
• C. They are part of the innate immune system.
• D. All of the above

Answer: (A)

Antibodies are produced by plasma cells, which are mature B cells that secrete antibodies after activation. When a B cell encounters its specific antigen and receives help from T cells, it differentiates into a plasma cell and releases large amounts of antibodies into the bloodstream. These antibodies are the effectors of the adaptive immune response, binding to antigens to neutralize pathogens and tag them for attack by other immune components. They are not part of the innate immune system, which includes barriers, phagocytes, and other immediate-acting defenses, and antibodies do not secrete antigens—they bind them. Because only this statement is true, it is the correct choice.