Biologics -- drugs derived from living sources such as humans, animals or microorganisms and grown in specially engineered cells as opposed to drugs synthesized using chemical reactions in the lab -- have a history that goes back into the 19th century.

Vaccines were first produced in the laboratory by Louis Pasteur in 1879, and insulins used in the treatment of diabetics -- another familiar biologic -- have been around for more than 75 years. Blood and blood components also are biologics.

But the biologics making such a big splash today -- drugs being used to treat a variety of medical illnesses and conditions that in many cases had no previous treatment available -- are a fairly recent outgrowth of research breakthroughs in the second half of the 20th century involving DNA, cell fusion, and genetics.

Today's biologics -- known as "large molecule" drugs -- are not only larger but far more complex than traditional "small molecule" drugs.

Small molecule drugs are typically composed of only 20 to 100 atoms and are taken orally. Aspirin, for example, has 21 atoms.

Small biologics, such as human growth hormones, are typically composed of about 3,000 atoms, and large biologics like antibodies are typically composed of 5,000 to 50,000 atoms. Genentech says Herceptin, its antibody used to treat metastatic breast cancer, has approximately 25,000 atoms.

Because of their large size, biologics typically are injected or infused into patients in a doctor's office.

The number of biologics is now expanding rapidly because of the remarkable growth of research in biotechnology and scientific advances such as completion of the Human Genome Project.

Today's biologics include recombinant therapeutic proteins, allergenics, somatic cells, antitoxins, and gene therapy, and biologics approved in recent years have revolutionized the treatment of heart disease, cancer, serious infections, arthritis, anemia, hemophilia, multiple sclerosis, and many other diseases.