Evolutionary immunology is an interdisciplinary field of research dedicated to understanding the origins, diversification, and adaptive evolution of immune systems across the tree of life. This field investigates the genetic, molecular, cellular, and systemic mechanisms underlying host defense and their historical trajectories, often employing comparative and phylogenetic approaches to elucidate how host-pathogen interactions have shaped immunological diversity and function. Its significance lies in revealing fundamental principles of immunity, informing our understanding of disease susceptibility and resistance, and guiding biomedical applications such as vaccine development.