One of the most important systems in the body of any species is the immune system. The immune system is a biological response that protects the body from dangerous pathogens that can cause harm or even death to the body. Without a well-developed immune system that best fits the species' body, an animal would have no chance of survival. An effective immune system has myriad mechanisms to protect the body from pathogens such as viruses and bacteria. The immune system has numerous mechanisms that allow it to protect the body from pathogens, such as mucus, phagocytes and effectors, or fevers. An animal's skin is one of the first external defenses against an infection, as it forms an impenetrable barrier of the epithelium, which are layers of keratinized cells. Epithelium is a term for the layers of cells that line the external surface and internal cavities of the body. Although only some species have skin cells made of keratin, keratin can be found in the feathers of a bird, the scales of a reptile, or the skin of humans and primates. Because of the many places keratin can be found, this shows us that the outer layer of keratin cells, or what humans have as skin, are not just a first-line defense for humans. The skin is not just an outer layer, but epithelia, layers of keratinized cells, line the respiratory, gastrointestinal, and urogenital tracts where infections are common. In internal surfaces, these internal surfaces are vulnerable to microbial invasions, known as mucous or mucous surfaces. The mucous surfaces are constantly wetted by the mucus secreted by their glands; these secretions fight microorganisms that become trapped and nest in the mucus. Mucus is a thick fluid layer that contains anti-paper mechanisms that the immune system uses to heal and protect the body. One way is mucus produced by glands in the body that capture pathogens that enter openings or vulnerable places in the body and expel them. Another mechanism is dexterous effector molecules that mark pathogens for phagocytes that ingest and then eliminate them. Finally there is the chemical response of temporarily maintaining a high temperature to kill pathogens that have entered and damaged the body, this is called a fever. The immune system has more than three mechanisms to fight infections and pathogens that enter the body, from stopping them before damage occurs, to healing the damage the body has suffered, to eliminating pathogens that cause physical harm. Works Cited1) Parham P 2009. Immune system [Internet]. 3. Quoted in May 2014 1