The immune system is one of the main systems of the human body. It protects the body from diseases and infections that could cause harm to a person's body. To protect the organism, a distinction is made between diseased cells and healthy cells (National Institute). The immune system distinguishes between these cells to determine whether the body is invaded by a disease or whether the body is well and healthy. It will fight foreign contaminants if they are invading the body potentially harming it. The immune system works to suppress cells, microbes, etc. unwanted. Without a properly functioning immune system, the human population would be constantly sick. The immune system acts as a defense system against unwanted microbes and pathogens in a healthy human. It prevents them from entering the body or, if they have already entered the body, it prevents them from causing harm to the body. The three main tasks of the immune system are to neutralize pathogens that have entered the body and then remove them, recognize and eliminate harmful substances from the body, and fight cells in the body that have been altered due to disease (PubMed Health). In order to determine what task the immune system must perform to fight the pathogen, it must be able to distinguish between self and non-self substances. The immune system is activated by non-self substances called antigens. Antigens attach to special receptor sites on defense cells that initiate cellular processes. If the body has already come into contact with the antigen, it will be able to respond more quickly (PubMed Health). There are two different parts of the immune system. Innate immunity is the most general type; for this reason it is also called nonspecific res...... middle of paper ......, MIKOŁUĆ, B., PIETRUCHA, B., & WOLSKA-KUŚNIERZ, B. (2013). Clinical and immunological analysis of patients with X-linked agammaglobulinemia: a single center experience. Central European Journal of Immunology, 38(3), 367-370.Primary Immunodeficiency Resource Center (n.d.). X-linked agammaglobulinemia. Retrieved April 10, 2014, from http://www.info4pi.org/aboutPI/index.cfm?section=aboutPI&content=syndromes&area=5&CFID=36419223&CFTOKEN=3244PubMed Health (n.d.). How does the immune system work? Retrieved April 8, 2014, from http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0010386/Stanford Children's Health (n.d.). X-linked agammaglobulinemia. Retrieved April 12, 2014, from http://www.stanfordchildrens.org/en/topic/default?id=x-linked-agammaglobulinemia-90-P01666VanPutte, C., Regan, J., & Russo, A. (2014) . Seeley's Anatomy and Physiology (10th ed.). NEW YORK, NY: MCGRAW-HILL.