Natural immunity

The innate immune system, also known as the non-specific immune system or in-born immunity system, is an important subsystem of the overall immune system that comprises the cells and mechanisms that defend the host from infection by other organisms. The cells of the innate system recognize and respond to pathogens in a generic way, but, unlike the adaptive immune system, the system does not provide long-lasting immunity to the host. Innate immune systems provide immediate defense against infection, and are found in all classes of plant and animal life.

The innate immune system is an evolutionarily older defense strategy, and is the dominant immune system found in plants, fungi, insects, and primitive multicellular organisms.

The major functions of the vertebrate innate immune system include:

Anatomical barriers include physical, chemical and biological barriers. The epithelial surfaces form a physical barrier that is impermeable to most infectious agents, acting as the first line of defense against invading organisms.Desquamation (shedding) of skin epithelium also helps remove bacteria and other infectious agents that have adhered to the epithelial surfaces. Lack of blood vessels and inability of the epidermis to retain moisture, presence of sebaceous glands in the dermis provides an environment unsuitable for the survival of microbes. In the gastrointestinal and respiratory tract, movement due to peristalsis or cilia, respectively, helps remove infectious agents. Also, mucus traps infectious agents. The gut flora can prevent the colonization of pathogenic bacteria by secreting toxic substances or by competing with pathogenic bacteria for nutrients or attachment to cell surfaces. The flushing action of tears and saliva helps prevent infection of the eyes and mouth.

Inflammation is one of the first responses of the immune system to infection or irritation. Inflammation is stimulated by chemical factors released by injured cells and serves to establish a physical barrier against the spread of infection, and to promote healing of any damaged tissue following the clearance of pathogens.

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