ABSTRACT
Section of Allergy, Pulmonary, and Critical Care Medicine, University of Wisconsin School of Medicine and Public Health, Madison, Wisconsin, U.S.A.
INTRODUCTION
The mammalian immune system evolved to protect individuals from various biological threats, which range from viruses and bacteria to multicellular parasites and neoplastic cells, by recognizing antigenic determinants that are distinct from self-expressed molecular moieties. Cells and tissues from individuals of the same species express a plethora of genetically determined major and minor antigens that differentiate self from nonself, and immune cells can be stimulated when exposed to nonself antigens from other individuals of the same species unless they are genetically identical, monozygotic twins. Although the immune system did not evolve to recognize and react to transplanted tissues, the ability that it has developed to resist infections and neoplasms by recognizing self versus nonself or ‘‘abnormal self’’ plays a key role in rejecting transplanted tissues and organs.
