affinity
Meaning of affinity: Binding tightly and doing something once bound are separate abilities, and a blocker can excel at the first while deliberately lacking the second.
Definition of affinity
Affinity is the strength with which two molecules stick to each other, such as a drug and its receptor or an antibody and the antigen it recognizes. High affinity means the pair binds readily and stays together, so an effect appears at low concentration. Low affinity means more of the substance is needed to occupy the same number of targets.
How is it measured?
Binding is not permanent. Molecules attach and let go continually, and affinity is really a statement about the balance between how quickly they find each other and how quickly they part. The usual measure is the dissociation constant, the concentration at which half the available targets are occupied at any moment. That number runs backward from intuition: the tighter the binding, the lower the concentration needed, so the better binder has the smaller value. Figures come from laboratory binding studies under controlled conditions of temperature and salt, and they describe a particular pair under those conditions rather than a fixed property of the drug alone.
How does it differ from the words it gets confused with?
| Term | What it describes |
|---|---|
| Affinity | How tightly the molecule holds to its target |
| Efficacy | How much effect it produces once it is bound |
| Potency | The concentration needed for a given effect, which folds both of the above together |
| Avidity | The combined strength when one molecule grips its target at several points at once |
| Selectivity | How strongly it prefers the intended target over similar ones |
Why does the distinction matter?
Because a substance can be excellent at binding and deliberately useless at acting. A blocking drug takes up residence on a receptor with high affinity and produces no signal at all, and that is the whole point of it: the tighter it holds, the more reliably it keeps the natural messenger out. Selectivity matters just as much, since a drug that binds its intended target tightly but also grips a similar one elsewhere in the body will produce unwanted effects there. Affinity also says nothing about how long a drug lasts in a person, which depends on how fast the body clears it. In the immune system the same property is deliberately improved over time: during a response, B cells refine the antibody they make so later versions bind their target far more tightly than the first ones, and laboratory made antibodies are engineered the same way so they can pull a scarce target out of a dilute mixture.
Used in a sentence
The compound demonstrated high affinity for the receptor in laboratory binding studies.
Good to know
Affinity describes how strongly a substance binds its target, a separate property from efficacy, the ability to produce an effect once bound, so a high-affinity substance is not necessarily one that produces a strong biological response.
Word origin
Where “affinity” comes from
From Latin affinis, meaning bordering on or related to, from ad-, meaning to, and finis, meaning boundary.