Antibody-antigen binding has been extensively used in many technological applications such as immunoassays, chemical analysis and bionanotechnology. The crucial element underlying these processes is the high molecular specificity. The binding of antigen to surface immobilized antibody is widely used to detect the antibody¿s bioactivity. Changes in the bioactivity reveal in situ structural conformation as well as interfacial packing density. The aim of this study is to determine antibody layer thickness and packing density by neutron reflection. Neutron reflection study is an important step towards the establishment of the relation between antigen binding capacity and the structure of the interfacially immobilized antibody.