Immunology is a branch of biology that covers the study of immune systems in all organisms. Cellular immunology is the study of the cells and molecules of an organism's immune system. The field involves studying how those different cells and molecules work together to provide a defense against different types of pathogens. To better understand cellular immunology, researchers study both healthy immune systems and those that are actively fighting off pathogens, comparing the differences and similarities of how the immune system's cellular physiology operates. Molecular immunology is a subfield of immunology that aims to examine immune processes at a molecular level. The immune system is the bodily system that responds to foreign entities, such as bacteria or other infectious agents in the body. The immune response that such a foreign entity triggers tends to be highly specific. The body produces antibodies that are specifically designed to target a particular antigen, or foreign body that triggers an immune response, just as a single lock tends to be matched to a single key. The field of molecular immunology exists to examine this and other aspects of immune response that are controlled at a molecular level. Immunology is a fast evolving subject, and attempt has been made in this work to keep it as much up-to-date as possible according to the requirement of the students and researchers in the field. This book reviews the principles of immunology and provides basic concepts of it by extracting the important information on immunology and peasants it in a concise, uncluttered fashion to prepare students for their courses.