Therapeutic agents derived from living cells or organisms, including vaccines, blood components, tissues such as cornea or skin and proteins such as MONOCLONAL ANTIBODY DRUGS. These agents often have complex structures which are difficult to characterise chemically and, because of the nature of how they are made, different batches of the same material may differ in some way. The monoclonal antibodies, in particular, have become of major importance in the last decade because of the ability to manipulate the molecular structure of these compounds to target specific pathological cells in the body, especially cancer cells.