Define recognition as it applies to protein receptors such as antibodies and proteins. Use the word ‘specificity’ in your definition.

Recognition is the ability of a protein to find and bind to a unique molecule or molecular structure.

Other molecules, even similar molecules are ignored unless there is an exact fit. This is called specificity.

The target molecule is called the ‘substrate’ or the ‘ligand’ and it is easily picked out of the crowd of other molecules even if it is present in very small quantities: down to 0.000000000001 Molar (10^-12 M) concentration.

(Self/Non-self recognition was not necessarily addressed in this question but it would be an example of two things:

o Specificity – the ability to pick out the many, many different molecules that form and function in our bodies; as well as the ability to pick out the even more numerous molecules that are not part of our bodies and differentiate these from ours.

o The multitude and diversity of receptor binding sites – Many, many, many, many different receptor binding sites must exist to account for the many specificities

What is the difference between a toxin and a toxoid? Use immune system or immunity in your answer.

A toxoid is an inactivated or attenuated toxin. A toxin is a poison made by other organisms which can make us sick or kill us. In other words, a toxin is toxic.

A toxin is both toxic and immunogenic

A toxoid is no longer toxic but it is still as immunogenic as the toxin from which it was derived.

The immune system is capable of responding to a toxoid and producing antibodies that will react with both the toxoid and the original toxin. When these antibodies react with the toxin they will inactivate it. Thus toxiods can be used as safe vaccines allowing us to build immunity to toxin-based diseases.