The Latin and Greek terms "immunis" are where the word "immunology" originates (Latin Immunis, forgiven, protected, Greek. Logos, word, science). It originally referred to the study of an organism's particular resistance to diseases. The immune response is beneficial from a biological standpoint since it aids the body in fighting illness, but it may also be damaging if it is overactive or targeted towards its own tissue. Despite having to adjust to quite different underwater settings than mammals, fish are not an exception. Fish are gill-equipped aquatic animals. The name "fish" is only used to refer to aquatic creatures whose jaws are members of the genus Chordata, and whose immune systems resemble those of mammals and humans. With the divergence of Cyclostomata and cartilaginous 450 million years ago, the first instance of adaptive immunity emerged because of adaptation to aquatic environment. The immune system of fish is mostly composed of innate and adaptive immunity, which are further divided into cells and bodily defensive mechanisms that will be described in this thesis.