Keyhole limpet hemocyanin (KLH) is a protein made by a shelled sea creature found along the coast of California and Mexico known as a keyhole limpet. It is a large protein that both causes an immune response and acts as a carrier for cancer cell antigens.

Cancer antigens often are relatively small proteins that may be invisible to the immune system. KLH provides additional recognition sites for immune cells known as T-helper-cells and may increase activation of other immune cells known as cytotoxic T-lymphocytes (CTLs).

Keyhole limpet hemocyanin (KLH) is one of a group of drugs called immune modulators, given as a vaccine to help the body respond to cancer. It’s used as an adjuvant in cancer treatment. Adjuvants are substances used to heighten the immune response to cancer antigens (toxins that induce an immune response from the body). Adjuvants can be attached as decoy substances in the body. These substances are weakened proteins or bacteria, which “trick” the immune system into mounting an attack both on the decoy and the tumor.