Antibodies are produced when the body is exposed to antigens macrophages receptor proteins

Option A

Explanation:

Antibodies are protective proteins that are released by immune system when any foreign agent enters into our body. These harmful foreign agents are known as antigens, in response to which antibodies are produced.  

B Lymphocytes are specialized white blood cells that produce antibodies when it comes in contact with the surface of the antigen which is different from the surface of other bio molecules with in our body. After interacting with antigen, B lymphocytes divides and form a clone of identical cells. This clone then secretes millions of antibodies into the bloodstream

Hence, option A is correct