*The zone phenomenon in antigen-antibody (Ag-Ab) reactions refers to the effect of antigen or antibody concentration on the formation of immune complexes.* - *It occurs in precipitation and agglutination reactions and is divided into three zones:* *1. Prozone (Antibody Excess Zone)* - There is an excess of antibodies compared to antigens. - Antigens are unable to effectively cross-link with antibodies, leading to no visible precipitation. *This can cause false-negative results in serological tests.* *Dilution of the sample can correct this and reveal the reaction.* *2. Equivalence Zone* - The optimal ratio of antigen and antibody is present. - Maximum cross-linking occurs, forming large lattice-like immune complexes, leading to visible precipitation. *This is the ideal condition for precipitation and agglutination reactions.* *3. Postzone (Antigen Excess Zone)* - There is an excess of antigens compared to antibodies. *The formation of immune complexes is hindered because antibodies get saturated with antigens, preventing proper lattice formation.* - Like the prozone effect, this can also cause false-negative results in serological tests. *Clinical Significance:* - The prozone effect can lead to false-negative results in tests like VDRL (for syphilis) and agglutination assays. - In cases of postzone, repeated testing after a few days (allowing antibody levels to rise) or diluting the sample can help achieve proper detection.