The substance must be able to alter its physical characteristic (for example, its color) in accordance to a change in pH. One example of this is litmus paper, which becomes red under acidic conditions and blue under basic conditions.
The nose, mouth, ears and eyes decide the entry of foreign particles. Little children mostly acquire it because mostly children put their hands or stick their tongues on nearby objects which are mostly infested with bacterias and pathogens, sometimes these particles are harmful and sometimes they aren't.
Not sure what you mean but here's what i think
1.They are substrate-specific, because it's a lock-and-key mechanism.
2. They are re-used
3. They can catalyze reactions
4. They decrease activation energy
5. They are proteins by nature.
<span>It
is important that antibodies have a symmetrical quaternary structure to create
two identical binding sites for antigens. An antibody is a relatively large
protein having a Y-shape. Plasma cells produce antibodies which are then used
by the immune system to fight off pathogens (e.g. bacteria and virus). The antibody
is able to recognize the antigen of the pathogen. It binds with it either to
neutralize it directly or “tag” the microbe for future attack by other parts of
the immune system. </span>