Amino acids work together, but each plays a different role in the body. Histidine, for example, is an essential amino acid that the body uses to make histamine. Histamine is part of an immune response to protect the body from foreign pathogens like allergens and bacteria. Without it, the body has no first line of defense, leaving your pet susceptible to many illnesses and infections. Histidine also helps make Hemoglobin, which carries oxygen through the bloodstream.