This is a useful measure to describe the strength of binding (or affinity) between receptors and their ligands.
Eg antibody+antigen = 10 -7 to 10 -11 M (remember 10 -7M is equivalent to 0.1mM or 100nM. If you are not confident with units, click here)
TCR+MHC/peptide = 10-6 M
For convenience, call the receptor A and it’s ligand B. The interaction of A and B will involve them being together (product) and apart (reactant), and the strength of this interaction will be the balance of between AB (or ‘on’) or A and B (or ‘off’).
This relationship can be written as an equation:
k1 (rate of dissociation) AB ---> A+B
k2 (rate of association) A+B --->AB
Constants (k1 and k2) are used so that we can convert this linkage ( ---> /<---) to an equals sign.
Therefore, if we know when
AB=A+B
it will tell us the strength of binding (ie is the complex AB more stable than its individual parts, A+B?).
At equilibrium,
k2[AB]=k1[A][B]
…remember, square brackets mean [concentration]
This can be rearranged to: k2 / k1 = [A][B] / [AB] = Kd
This dissociation constant, Kd, indicates the strength of binding between A and B in terms of how easy it is to separate the complex AB (dissociation or ‘off rate’).
If a high concentration of A and B is required to form AB, this indicates that the strength of binding is low. The Kd would therefore be higher (mM rather than nM) as more of A and B are required to form AB.
It follows that the smaller Kd, the stronger the binding. So 10-6M (or 1mM) indicates weak binding compared to 10-9M (or 1nM).
Therefore, we can measure that TCR and MHC/peptide have a relatively low strength of binding compared to lots of other biological receptor ligands (eg antibody and antigen; adhesion molecules).