The antigen-antibody immunological reaction used to be regarded as typical of immunologica
The reluctance to abandon this hypothesis, however well it explains specific processes, impeded new research, and for many years antigens and antibodies dominated the thoughts of immunologists so completely that those immunologists overlooked certain difficulties. Perhaps the primary difficulty with the antigen-antibody explanation is the informational problem of how an antigen is recognized and how a structure exactly complementary to it is then synthesized. When molecular biologists discovered, moreover, that such information cannot flow from protein to protein, hut only from nucleic acid to protein, the theory that an antigen itself provided the mold that directed the synthesis of an antibody had to be seriously qualified. The attempts at qualification and the information provided by research in molecular biology led scientists to realize that a second immunological reaction is mediated through the lymphocytes that are hostile to and bring about the destruction of the antigen. This type of immunological response is called cell- mediated immunity.
Recent research in cell-mediated immunity has been concerned not only with the development of new and better vaccines, but also with the problem of transplanting tissues and organs from one organism to another, for although circulating antibodies play a part in the rejection of transplanted tissues, the primary role is played by cell-mediated reactions. During cell-mediated responses, receptor sites on specific lymphoeytes and surface antigens on the foreign tissue cells form. a complex that binds the lymphocytes to the tissue. Such lymphocytes do not give rise to antibody- producing plasma cells but themselves bring about the death of the foreign-tissue cells, probably by secreting a variety of substances, some of which are toxic to the tissue cells and some of which stimulate increased phagocyte activity by white blood cells of the macrophage type. Cell-mediated immunity also accounts for the destruction of intracellular parasites.
The passage suggests that scientists might not have developed the theory of cell-mediated immunological reactions if______.
A.proteins existed in specific group types
B.proteins could have been shown to direct the synthesis of other proteins
C.antigens were always destroyed by proteins
D.antibodies were composed only of protein