All higher organisms have an immune system that constitutes a biological defense mechanism to protect the body from pathogens and foreign substances. It consists of a complex network of different organs, cell types and molecules that recognizes and fights harmful substances and pathogenic microorganisms. Invertebrates rely exclusively on the innate immune system, which protects the integrity of the organism from pathogen attacks through various barriers and other defense mechanisms. The innate immune system emerged early in the history of life and its defense reactions are generally considered unspecific. Later in evolution, vertebrates evolved a more complex and adaptable system in addition to the innate immune system, the adaptive immune system. It is also called the acquired immune system and is capable of highly specific responses to different antigens and immunological memory and is therefore considered extremely specific.
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Source: Phys.org