Immunology is the branch of biomedical science that focuses on the study of the immune system, its functions, and the role it plays in protecting the body from pathogens and other harmful invaders. It encompasses the recognition of foreign molecules and the activation of mechanisms to neutralize or eliminate these threats, while maintaining tolerance to self-tissues. Immunology is an evolving field that links various disciplines, including microbiology, genetics, and molecular biology, in understanding how the body defends itself against diseases.

The immune system is a complex network of cells, proteins, and organs that work together to provide protection against infections and malignancies. It is composed of two major arms: innate and adaptive immunity. Innate immunity is the first line of defense, offering a rapid but non-specific response to a wide range of pathogens. In contrast, adaptive immunity is more specialized, offering a delayed but highly specific response to pathogens, with the ability to "remember" previous encounters and respond more effectively in the future.

Historically, the field of immunology has been shaped by significant discoveries that have helped define the immune system's components and their roles. The work of pioneers such as Louis Pasteur, Edward Jenner, and Robert Koch laid the foundation for our understanding of immunity, particularly in relation to vaccines and infectious diseases. As research advanced, immunology expanded into areas like autoimmunity, immunodeficiencies, and cancer immunology, offering new insights into disease mechanisms and therapeutic strategies.