[单项选择] On Antibodies Substances foreign to the body, such as disease-causing bacteria and viruses and other infectious agents, are recognized by the body s immune system as invaders. Our natural defenses against these infectious agents are antibodies, proteins that seek out the antigens (抗原) and help destroy them. Antibodies have two very useful characteristics. First, they are extremely specific; that is, each antibody binds to and attacks one particular antigen. Second, some antibodies, once activated by the occurrence of a disease, continue to confer resistance against that disease. Classic example are the antibodies to the childhood diseases of chickenpox(水痘) and measles. The second characteristic of antibodies makes it possible to develop vaccines. A vaccine (痘苗) is a preparation of killed or weakened bacteria or viruses that, when introduced into the body, stimulates the production of antibodies against the antigens it contains. It is
A. disease-causing bacteria
B. disease-causing viruses
C. antigens
D. protein