Sophie
Antibody affinity refers to the strength with which the epitope binds to an individual paratope (antigen-binding site) on the antibody. High affinity antibodies bind quickly to the antigen, permit greater sensitivity in assays and maintain this bond more readily under difficult conditions. Low affinity antibodies, by contrast, bind weakly to the antigen and often do not detect the antigen in-vivo or in assays.
Maybe you can get more help from Leading Biology.
Roger the Mole
see https://www.pacificimmunology.com/resources/antibody-introduction/antibody-affinity-and-affinity-maturation/
[This is not really a question about Chemistry. You'd likely get more and better answers by moving it to Biology or Medicine.]