Antibodies are high value therapeutic, diagnostic, biotechnological and research tools. Combinatorial approaches to antibody discovery have facilitated access to unique antibodies by surpassing the diversity limitations of the natural repertoire, exploitation of immune repertoires from multiple species and tailoring selections to isolate antibodies with desirable biophysical attributes. Here we present the crystal structure of the disulphide-constrained antibody (scFv B8) in complex with its cognate antigen, prostate specific antigen (PSA). PSA, a serine protease, is a member of the kallikrein family and the gold standard prostate cancer biomarker (Gilgunn et al. 2013). In this crystal structure we have captured the antibody interaction with free PSA (fPSA) in both active and inactive forms. These structures, in combination with analysis of key mutants, allow a comprehensive evaluation of the contribution of these cysteine-containing loops. These structures cast light on the unique structural features of chicken antibodies and contribute further to our collective understanding of the unique mechanisms of diversity and biochemical attributes that render the chicken repertoire of particular value for antibody generation. Accruing knowledge of antibody structure, function, repertoire and origin is crucial for a number of important applications including man-made, knowledge-based, repertoire synthesis and antibody-based drug discovery. These structures give further structural endorsement to the active mechanisms of diversification employed by the chicken repertoire.