Antigen–Antibody Interaction
Antigen–antibody binding is specific, reversible, and driven by non‑covalent forces (hydrogen bonds, electrostatic forces, hydrophobic interactions, and van der Waals forces). This interaction exists in a dynamic equilibrium between free antigen, free antibody, and the antigen–antibody complex. The stability and strength of this binding are expressed through the association and dissociation constants, which help quantify how strongly an antibody recognizes its antigen.
Fundamental Binding Reaction
The basic reversible reaction representing antigen–antibody interaction is:
Where:
- [Ab] – free antibody concentration
- [Ag] – free antigen concentration
- [AbAg] – antigen–antibody complex concentration
This equilibrium allows complexes to form and dissociate continuously.
Association Constant (Ka)
The association constant measures the strength of complex formation.
Where:
- kon – rate of association (complex formation)
- koff – rate of dissociation (complex breakdown)
Key points:
- High Ka ⇒ strong binding
- Low Ka ⇒ weak binding
- Reflects how readily Ab and Ag come together to form Ab–Ag
Dissociation Constant (Kd)
The dissociation constant measures the tendency of the complex to fall apart.
Key points:
- Low Kd ⇒ strong interaction (complex stays intact)
- High Kd ⇒ weak binding
- Kd is the reciprocal of Ka
Affinity
Affinity refers to the strength of binding between a single antigenic epitope and a single antibody binding site.
It can be expressed as:
Important notes:
- Applies best to monovalent interactions (one paratope–one epitope)
- Higher affinity = tighter, more stable binding
Avidity
Avidity is the overall binding strength resulting from multivalent interactions.
Because most antibodies have two paratopes, and many antigens have multiple epitopes, several binding reactions occur simultaneously.
Avidity reflects:
- The combined strength of these multiple interactions
- The stability of the entire complex, not just one site
- Why IgM (with 10 binding sites) has high avidity despite moderate affinity
In summary:
- Affinity = strength of one Ab–Ag interaction
- Avidity = strength of all interactions combined
This simple equation shows that:
- Antibody and antigen meet and bind
- They form a complex
- The complex can dissociate again
These values help determine:
- How strong the immune response will be
- How easily antigen can be detected in assays
- How much antibody is needed for effective binding
Quick Concept Snapshot
- Ka high → strong binding
- Kd low → strong binding
- Affinity → single site strength
- Avidity → full multivalent strength