Nearly all organisms have some kind of immune system to
protect their body from damage. Bacteria have a rudimentary immune system in the form of enzymes that protect against viral infections. Other basic immune mechanisms evolved in ancient plants and animals and remain in their modern descendants. These mechanisms include phagocytosis, antimicrobial peptides called defensins, and the complement system.

Your body has even more sophisticated defense systems.  Complex and pervasive, your immune system is found in your skin, bone marrow, bloodstream, thymus (a small organ located in the upper chest), lymphatic system, eye, spleen, and mucosal tissue (including the respiratory tract, and the gut). Perhaps most importantly, you have adaptive (or acquired) immunity, which creates an immunological memory when you re-encounter the same pathogen. So, unlike many other organisms, your body “remembers” the pathogen it already met and responds in an enhanced way.