Immunology is a diverse and growing discipline that can be defined as the study of the tissues, cells and molecules involved in host defence mechanisms. The immune system is the system of specialized cells and organs that protect an organism from outside biological influences using several strategies, of which early activation of defence is one of the most important. The mechanisms used by the immune system to counterattack microbes often rely on the immediate recognition of microbes, or of cells that have been affected by the infection of bacteria and viral infections, for example. The immune system also performs surveillance of tumor cells, and immune suppression has been reported to increase the risk of certain types of cancer. Disorders of the immune system can result in a multitude of diseases. When the immune system is less active than normal, immunodeficiency occurs resulting in recurring and life-threatening infections. In contrast, autoimmunity results from a hyperactive immune system attacking normal tissues as if they were foreign organisms.
The immune system is composed of two major parts: the innate immune system and the adaptive immune system. The innate, or non-specific immune system comprises the cells and mechanisms that defend the host from infection by other organisms in a non-specific manner, and is present in all living organisms and comes into play immediately or within hours of an antigen's appearance in the body. Innate responses use phagocytic cells (neutrophils, monocytes and macrophages) and natural killer cells that use a primitive non-specific recognition system that allows them to bind, internalize and kill a variety of micro-organisms and exogenous molecules. The molecular components of innate reponse include complement, acute-phase proteins and cytokines such as interferons. The adaptive immune system however, effectuates an antigen-specific immune response and confers long-lasting or protective immunity to the host and is only found in vertebrates. This kind of immune response involves antigen-presenting cells (APCs), B and T cells. APCs are a heterogenous population of leukocytes with a immunostimulatory function[1].

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