Our immune system protects us against a variety of infectious micro-organisms, such as bacteria, viruses and fungi that are found in the environment. Intact skin forms an effective barrier against invasion. However, despite our vigilance, other types of proteins and microbes can gain access through damage and penetrate mucous membranes of the lungs, nasopharynx, gastrointestinal and urogenital tracts. Once inside the body, the immune system is able to recognise these microorganisms as foreign through its enormous variety of cells and molecules and thereby mount a response to eliminate or neutralise pathogens. Our immune system is therefore crucial to our survival. It is currently divided into two categories which are innate (non-specific) immunity and acquired (specific) immunity.