Antigen-Antibody Interaction Essay

Antigen-Antibody Interaction

Antigen and antibodies are complement to each others. Different antibodies are attracted to the specific kind of antigens because of the affinity between the two. The association between the two depends on different kind of non-covalent weak bonds that are easily breakable hydrogen bonds. They are attracted by electrostatic forces, van der Waals forces and hydrophobic forces. Antigens are either proteins, polypeptides, lipids, nucleic acids or polysaccharides. Haptens coupled with carrier protein can also work as antigen.

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Saturation Curve


50  60  70  80  90  100      200     300      400     500       600     700       800      900      100

Specific Binding





Antigens and antibodies are complementary to each other and each antibody binds to specific kind of antigens. The specific antibody binding site of antigen is called epitope. The epitope is usually made of amino acid residues or monosaccharides. Multivalent antigens have multiple epitopes hence several binding sites of different kind of antibodies. Interaction between multivalent antigens and antibodies is more stabilized and strong. Any binding between antigen and antibody is reversible. As the results of experiment show that saturation curve is never reached and multivalency can cause steric difficulties. Their bimolecular interaction depends on thermodynamic principles.

Ka = [Ab-Ag]

Ka = [Ab:(c)1] / [Ab] * c

Where Ka = Affinity constant

            Ab = free Antibody

            c= free antigen

            Ab:(c) and Ab-Ag = Antigen and antibody complex

Affinity of antibody with a particular antigen is determined by the affinity constant (Ka). Antigen and antibody complex (in molar concentration) is divided by the product of free antigen and antibody.

The time required to attain equilibrium depends on the affinity of antibody for antigen and also on the diffusion rate. The affinity constant varies depending upon the type of antibody-antigen interaction. The results show that type 1 antibodies have the ka ratio of 1 and antigen-antibody are equally attracted while type 2 antibodies have more affinity for antigen. Affinity constant can also change with temperature, pH of the liquid and the type of solvent used. In the dialysis experiment the Scatchard plots reveal valence of 2 and cross reactivity between antigen and antibody.




Scatchard Plot

10000      20000     30000      40000     50000     60000    70000      80000    90000    100000


X-intercept =Bmax = 93857

Slope = -1/Kd

Y-intercept = Bmax / Kd

If affinity constant is high it means that affinity of antibody for antigen is also high and more antibodies are attracted towards antigen binding sites. If affinity constant is low it means that affinity of an antibody for an antigen is low and fewer antigen binding sites are occupied by antibodies. Avidity determines the stabilization of the antigen-antibody complex. Cross reactivity is the binding of antibody to the epitope of another antigen. This only happens when the stabilization of the antigen-antibody complex is not very strong or specificity of antibodies is not very strong for particular type of antigens but it can bind with different antigens with somewhat identical epitopes.

The graph between bound antigen and free antigen shows that saturation curve has not reached to the saturation level.

Radioactive small antigen = 1015 cpm/mole (counts per minute per mole)

c = free ligand (antigen, hapten) equilibrium concentration

n = valence (= 2 for these antibodies)

Ka1, Ka2 = equilibrium association constants

r1, r2 = moles bound ligand (antigen) per mole antibody (Ab1 or Ab2, respectively).

Equilibrium Reactions


[Ab1] + c <=> [Ab1:(c)1],  Ka1

[Ab1:(c)1] + c <=> [Ab1:(c)2],  Ka1


[Ab2] + c <=> [Ab2:(c)1],  Ka2

[Ab2:(c)1] + c <=> [Ab2:(c)2],  Ka2

Association constant

Ka1 = 1.00E+09 = [Ab1:(c)1] / [Ab1] * c = [Ab1:(c)2] / [Ab1:(c)1] * c

Ka2 = 1.26E+08 = [Ab2:(c)1] / [Ab2] * c = [Ab2:(c)2] / [Ab2:(c)1] * c

Scatchard equations:

r1 / c = Ka1 * ( 2 – r1 )

r2 / c = Ka2 * ( 2 – r2 )

rave = ( r1 * ratio + r2 ) / ( 2 * ratio + 2)

r1 = moles ligand (antigen) bound per mole of Ab1

r2 = moles ligand (antigen) bound per mole of Ab2

ratio = 1.00 = [Ab1]total / [Ab2]total

Results clearly show that the affinity constant for antibody 1 is equal to 1 while that of antibody 2 is a bit high meaning second antibody has higher affinity for antigens. During the dialysis experiment as the concentration of antibodies was kept constant the concentration of radioactive antigens was changed and simultaneously bound antigen activity was recorded. The antibody 2 showed higher level of affinity with antigen as compared to antibody 1. Radioactive small antigen was 1015cpm/mole. Hence this shows high level of antigen activity and its interaction with antibody 2 is stronger than antibody 1. That is the interactive bonds between antigen and antibody 2 is stronger as compared to the complex made with antibody 1.


Antigen antibody interaction.

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