Question

how attenuated viruses or bacteria act as antibody when introduced into body...

Answer 1

Attenuated viruses have denatured proteins which in turn have specific epitopes. These epitopes are result of specific arrangement of amino acids. If these epitopes are new to immune system, it will form antibodies against those epitopes. Many times these antibodies will be active against live viruses too.

Answer 2

The body's immune system recognizes foreign invaders not by what's inside, but by what's outside (the invader's protein coat or cell wall). The invading viruses and bacteria, which are non-functional on the inside, have specific proteins on their harmless outer surfaces that act as antigens to the immune system - meaning that the immune system will detect these invaders' surface proteins and recognize them as foreign and unwanted. The immune system will then remember this for future use, and produce antibodies that bind to the specific antigens. So in reality, the invaders DO NOT act as antibodies, but their surfaces do act as antigens. When the invader attacks again, the body produces the antibodies (from memory) that cling to the invaders, acting as a tag. The white blood cells then recognize the antibody "tag" and eat whatever is tagged (in this case, the invaders).