For immunological reasons, the choice for carrier proteins usually ends at bovine serum albumin (BSA), ovalbumin (OVA), or keyhole limpet hemocyanin (KLH):
When the cost is important, BSA or OVA are usually chosen. Albumins have good solubility characteristics and are well-described molecules. BSA has high nonspecific binding, making it ideally suitable for use in immunoassay capture antigens. However, BSA is not generally used in immunogens, because it crossreacts immunologically with other serum albumin proteins.
OVA has been successfully used in immunogens as well as capture antigen, though it does not crossreact as strongly with serum albumins as BSA.
In fact, KLH is the preferred choice for good immunogenicity, attributing to its relatively distant phylogenetic removal from the vertebrates generally used for anti-body production. KLH functions for the mollusk as an oxygen-transport metalloprotein, existing as a family of soluble oligomers in the animal’s hemolymph. It is usually purchased as a green-gray lyophilized powder. Dried KLH is virtually insoluble in water, which is much better in PBS.
Other invertebrate proteins are similarly immunogenic in experimental animals, which are often considerably less expensive.