KLH (Keyhole Limpet Hemocyanin) is a copper-containing oxygen carrier occurring freely dissolved in the hemolymph of many mollusks and arthropods. KLH is used extensively as a carrier protein in the production of antibodies for research, biotechnology, and therapeutic applications. Haptens are substances with a low molecular weight such as peptides, small proteins, and drug molecules that are generally not immunogenic and require the aid of a carrier protein to stimulate a response from the immune system in the form of antibody production. KLH is the most widely employed carrier proteins for this purpose. It is an effective carrier protein for several reasons. Its large size and numerous epitopes generate a substantial immune response, and the abundance of lysine residues for coupling haptens, allows a high hapten: carrier protein ratio increasing the likelihood of generating hapten-specific antibodies. KLH may also be a challenging molecule to work with because of its propensity to aggregate and precipitate. Aggregates remain immunogenic but limit the ability to conjugate haptens and are difficult to manipulate in the laboratory. A high-quality KLH preparation with a clear opalescent blue color is the best indicator of KLH solubility. KLH is being tested as a therapeutic vaccine for a variety of cancers, including non-Hodgkins lymphoma, cutaneous melanoma, breast, and bladder cancer. These vaccines use specific tumor-associated antigens (Haptens) conjugated to KLH to stimulate the body’s immune system to generate anti-tumor immune responses which can destroy tumor cells.