Oswald Theodore Avery studied strains of pneumococcus of the genus Streptococcus in the US in the first half of the twentieth century. This bacterium causes pneumonia, a common cause of death at the turn of the twentieth century. In a 1944 paper, Avery demonstrated with colleagues Colin Munro MacLeod and Maclyn McCarty that deoxyribonucleic acid, or DNA, instead of protein, formed the material of heritable transformation in bacteria. Avery helped untangle some of the relationships between genes and developmental processes.

Ilya Ilyich Mechnikov studied phagocytes, immune function, and starfish embryos in Europe during the late nineteenth and early twentieth centuries. Mechnikov adopted the French form of his name, Élie Metchnikoff, in the last twenty-five years of his life. In 1908, he won the Nobel Prize in Physiology or Medicine with Paul Ehrlich for their contributions to immunology. Mechnikov discovered phagocytes, immune cells that protect organisms by ingesting foreign particles or microorganisms, by conducting experiments on starfish larvae. He then developed a theory of the cellular process involving phagocytes, known as phagocytosis, to explain how inflammation is a part of the self defense system found in both vertebrates and invertebrates. His experimental work was part of the tradition of evolutionary embryology, which emerged in the decades following the publication of Charles Darwin's On the Origin of Species in 1859, and was influenced by Ernst Haeckel's concept of the biogenetic law.

Robert Alan Good was an American physician and scientific researcher who explored the cellular mechanisms of immunity. His research and discoveries earned him the label of "father of modern immunology." Though his work in immunology is considered his greatest scientific achievement, Good is also well known for his work with tissue engineering. From his research on immunology, Good was able to perform the first successful allogeneic (donor and recipient are unrelated) bone marrow transplant. A bone marrow transplant is a form of hematopoietic stem cell transplant in which hematopoietic stem cells are infused into a patient to treat various diseases of the blood including some autoimmune and inherited conditions, and cancer. Following his success with bone marrow transplants, Good established a bone marrow transplantation program for children at the University of South Florida.