To produce polyclonal antisera, animals (typically rabbits or sheep) are immunised with a particular antigen (e.g. FLCs). As a result, multiple B-cells are activated to multiply, differentiate and produce antibodies, which each target a specific epitope on the antigen (Figure 8.1). As a result, serum collected from the immunised animal contains polyclonal antibodies that collectively, will demonstrate a range of different specificities and epitope affinities (Figure 8.2). For Freelite assays, the repertoire of polyclonal antibodies is further increased through blending a large array of polyclonal antisera that have been raised in different sheep and against a wide variety of different monoclonal FLCs (Chapter 5). Prior to use in the assays, the polyclonal sheep antibodies are purified by positive- and negative-affinity chromatography against a diverse range of FLCs and intact immunoglobulins. This is necessary to produce the high-specificity, high-titre and high-affinity antibodies that are required for Freelite sFLC assays.