Humans inhabit an environment teeming with microbes and their toxins. The omnipresent threat of infection and the emergence of new infectious diseases has catapulted immunology to the forefront of modern biomedical science. Immunology is the study of the cellular and biochemical mechanisms which protect us from these threats. Infectious agents such as human immunodeficiency virus which causes AIDS and immune-based diseases such as asthma have increased the public awareness about the importance of immunology in examining the causes and treatments of these diseases. Immunology is unique amongst the natural sciences in that it covers and integrates aspects of modern biological science including cell and developmental biology, biochemistry, genetics, and molecular biology.<br><br>Using approaches from many disciplines and techniques as diverse as recombinant DNA, flow cytometry, physical chemistry, mouse transgenesis, and targeted mutation in mice, immunologists have made major advances and striking discoveries leading to an ever-growing understanding of the immune system. We continue to explore questions such as how the immune system distinguishes self from non-self, how antibodies, T cells, and natural killer cells specifically recognize and distinguish millions of different foreign invaders, why some individuals are more susceptible to the development of autoimmune diseases, and how the immune system rejects transplanted organs but often fails to reject tumors. In addition, immunologists study questions of broader biological significance including how gene expression is regulated during animal development, the biochemistry and regulation of gene rearrangement, the mechanism and function of immune surveillance, and the control of programmed cell death.