Final answer:
A plasma cell is responsible for secreting millions of identical antibodies, matched to the specificity of the antigen receptor on its parent B-lymphocyte. These antibodies are crucial for the body's immune response, forming complexes with antigens to facilitate the destruction of pathogens.
Step-by-step explanation:
The single word missing from the statement is plasma cell. A plasma cell secretes millions of molecules of antibody, but they are all identical and will be related to the single specificity of antibody used as the antigen receptor on the B-lymphocyte from which the plasma cell was derived. These plasma cells are the main effectors of the humoral immune response, acting as antibody factories after their differentiation from B-lymphocytes. Once activated by an antigen, B-lymphocytes proliferate and differentiate into plasma cells, which then produce large quantities of antibodies with the same antigen specificity as the original B cell receptors (BCRs). This process ensures that the immune system can produce a targeted and effective response to specific pathogens. The antibodies, also known as immunoglobulins, bind to antigens to form an antigen-antibody complex that flags the antigen-bearing cell for destruction. The first antibodies to be secreted during an immune response are usually of the IgM class, which initially are not as stable in their antigen binding as subsequent IgG antibodies produced upon re-exposure to the same pathogen.