Q.4 Which of the following statements about antigen-antibody (Ag-Ab) complexes is (are) TRUE?
(A) Hydrogen bonds and van der Waals forces participate in Ag-Ab interactions
(B) Ionic bonds and hydrophobic bonds participate in Ag-Ab interactions
(C) The combined strengths of all interactions between a single antigen binding site on an antibody
and a single epitope is called avidity
(D) Antibody elicited by one antigen can cross react with an unrelated antigen

Options A and B are true statements about antigen-antibody (Ag-Ab) complexes.

Option Analysis

(A) Hydrogen bonds and van der Waals forces participate in Ag-Ab interactions

This statement is correct. Antigen-antibody binding relies on non-covalent forces, including hydrogen bonds formed between electronegative atoms like oxygen or nitrogen on the antigen and antibody, and van der Waals forces that stabilize close-contact surfaces through transient dipoles.

(B) Ionic bonds and hydrophobic bonds participate in Ag-Ab interactions

This statement is also correct. Ionic (electrostatic) bonds arise from attractions between oppositely charged groups, such as lysine’s NH4+ and carboxylates, while hydrophobic interactions drive non-polar regions together, excluding water and contributing up to 50% of binding strength.

(C) The combined strengths of all interactions between a single antigen binding site on an antibody and a single epitope is called avidity

This statement is false. The strength between one binding site (paratope) and one epitope defines affinity, not avidity; avidity measures the total strength from multiple binding sites on the antibody (e.g., IgM’s 10 sites) interacting with multivalent antigens.

(D) Antibody elicited by one antigen can cross react with an unrelated antigen

This statement is false. Cross-reactivity occurs only with related antigens sharing identical or similar epitopes, not truly unrelated ones; unrelated antigens lack structural similarity for binding.

Antigen-antibody complexes form the cornerstone of immune recognition, where specific non-covalent forces drive binding in Ag-Ab interactions. For CSIR NET aspirants, understanding these complexes—key phrase: antigen-antibody complexes—is essential for immunology questions on binding forces, avidity, and cross-reactivity.

Key Forces in Ag-Ab Interactions

Ag-Ab interactions involve multiple weak non-covalent bonds for specificity and reversibility. Hydrogen bonds and van der Waals forces provide precise fitting, while ionic bonds (electrostatic attractions) and hydrophobic bonds exclude water from non-polar regions. These cumulative forces ensure tight, yet dynamic, complex formation vital for immune responses.

Affinity vs Avidity in Antigen-Antibody Complexes

Affinity measures binding strength at a single antigen binding site and epitope, determined by equilibrium constants. Avidity, however, reflects overall strength from multiple sites on bivalent (IgG) or multivalent (IgM) antibodies engaging multimeric antigens. This distinction clarifies why IgM shows high avidity despite lower individual affinity.

Cross-Reactivity in Ag-Ab Binding

Antibodies from one antigen rarely bind unrelated antigens; cross-reactivity requires shared epitopes with structural similarity, often seen in polyclonal responses or allergies. Unrelated antigens trigger no reaction due to epitope mismatch.