The immune system is the third line of defense which perform special defenses in the human body. It comprises mechanisms and agents which target particular antigens (Ags) which is known or unknown by humans. An antigen is any particle, ordinarily a protein or polysaccharide can be prevented from entering into body by special defenses.
The immune system defends the body against disease-causing microorganisms. It comprises the two levels of strength, the non-specific defenses, and the specific defenses. The non-specific protection, such as the skin and mucous layers, prevent microorganisms from penetrating into the body.