Immunity is a physiological process that allows an organism to maintain homeostasis (internal equilibrium) in the presence of harmful agents, be biological (pathogens) or chemical (contaminants, radiation). The immune system is composed of a variety of cells and molecules each with a specific function. Main types of cells are: leucocytes such as neutrophils, linfocytes, eosynofils, basofils, mastocytes, monocytes, and macrophages. Main molecules: antibodies, antigens, histamine, etc.
There are physical barriers such as mucose, however, when a pathogen surpasses them, their innate immune system enters into action. It is characterized because doesn't confer long-lasting immunity and is none specific. There is a second type of immune response, which is the adaptive immune system, this one is quite specific, and leads to immunological memory.
Examples
Physical barrier: mucose in the nose
innate immune system: Fever during infectious disease.
adaptive immune system: response triggered by a vaccine, the antigen is recognized and stored in cellular memory.