Antibody affinity is the strength of the reaction between a single antigenic determinant and a single combining site on the antibody. It is the sum of the attractive and repulsive forces operating between the antigenic determinant and the combining site of the antibody.
Affinity is the equilibrium constant that describes the Antigen-Antibody reaction. Most antibodies have a high affinity for their antigens
Avidity is a measure of the overall strength of binding of an antigen with many antigenic determinants and multivalent antibodies. Affinity refers to the strength of binding between a single antigenic determinant and an individual antibody combining site whereas avidity refers to the overall strength of binding between multivalent antigens and antibodies. Avidity is influenced by both the valence of the antibody and the valence of the antigen. Avidity is more than the sum of the individual affinities.