An antibody can be functionally defined by the antigen it binds to, as long as the antigen is clearly defined in the claims. If the antigen is defined by a protein sequence, no sequence variability and no open language (e.g. an antigen comprising …) can be used in the definition of the antigen. Otherwise the subject-matter of the claim will be considered to lack novelty over any known antibody because existing antibodies will bind to the undefined region of the target antigen.
Examples of accepted antigen-defined antibody claim wording are:
An antibody can also be defined by its ability to bind to a well-defined antigen in combination with a negative feature as for example: "Antibody binding to antigen X and not binding to antigen Y".