The immune system is a system for self-recognition and maintaining homeostasis. It is an extremely complex network that extends throughout the body, and is able to recognize and defend the organism against a theoretical infinity of challenges. The defence against pathogens is mediated by early stages of innate immunity and late adaptive immune responses. Macrophages and neutrophils, along with NK cells, complement and defensins participate in the innate immune mechanism, which all together constitute the first line of defence. They all need a basic capacity to distinguish between self and foreign, and danger or non-danger signals with the involvement of toll-like receptors (TLR). By picking up, processing, and presenting antigens, macrophages constitute the link between non-specific and specific branch of the immune system which mainly consists on the different subpopulations of lymphocytes and their products. This specific branch constitutes the adaptive response, which needs the previous contact with antigens and it is developed through a clonal proliferation of lymphocytes (T, cytotoxic lymphocytes, and B lymphocytes) resulting in the release of specific antibodies.

When the immune system does not work properly, the consequences could be serious. Immune system disorders include:

1. Allergy and asthma:  inappropriate immune responses to usually harmless substances
2. Immune deficiency diseases:  disorders with decreased resistance to infections
3. Autoimmune diseases:  diseases where the immune system attacks by mistake their own cells and tissues