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Full Version: Immunity: How your Body Protects Itself from Infections
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What is immunity and types of immunity:

Human body is a store house of wonders and astonishing features and functions making it an interesting and curious subject to understand and deal with. Made up of millions of cells, the role of each cell, tissue and organ right from the date of manufacture (embryo) till expiry (death) is quite amazing. Our body takes care of itself very well and also knows how to protect itself from molecules foreign to the body. In this regard our body has a well developed defensive system always ready to fight with the pathogen entering the body called as the immune system and the resistance towards a pathogen is called as the immunity. Here let us discuss the two types of immunity, the innate immunity and the acquired immunity and the phenomenon of defense in both these types of immunity.

Innate Immunity: The inbuilt immunity of an individual based on his or her genetic makeup is called as the innate immunity in contrast to the prior exposure to antigen to develop immunity. The innate immunity is seen either at species level or racial level or individual level. Species innate immunity as the name implies is the immunity shown by all individuals of the same species to the pathogens of different species. Racial immunity is the term applied to the difference in resistance shown by races or groups belonging to the same species. The individual innate immunity is the difference in resistance among individuals belonging to the same race or group.

The mechanism of defense in innate immunity is categorized into three types based on the type of protective barriers involved. They are the anatomical barrier, the physiological barrier and the phagocytic or endocytic barrier. The anatomical barriers protecting the body from various pathogens is the skin and the mucous membrane lining the respiratory tract, digestive tract etc. Skin, the large blanket covering all the organs of the body spread from head to toe is the first layer of defense. The hair follicles on the skin, the oil and sweat glands present in skin all forms the components of defensive mechanism of the skin. The mucous membrane lining the nasal area plays an important role in trapping the pathogens trying to enter the body. The mucous membrane lining the respiratory tract has cilial structure which pushes the foreign body out of the system, thus protecting the individual. Also the mucous membrane of the GI tract has similar function in tackling the pathogen. The tear, saliva and mucous are the secretions engaged in antibacterial and antiviral activity.

Looking into the physiological barrier they are the pH, temperature and chemical mediators. Temperature acts as an immunity agent by showing direct effect on the pathogen. For example chicks with high body temperature show resistance to anthrax. Likewise pH also contributes to immunity. The acidic nature of the stomach restricts and eliminates the growth of various pathogens. One example of chemical mediators as barriers is well indicated by the bacterial cell wall cleaving property of the hydrolytic enzyme, the lysozyme. The phagocytic or endocytic mode of defense involves the role of cells in forming structures to engulf and digest the pathogen, bacteria for example.

Acquired Immunity: Acquired immunity is the specific resistance developed by the body only on exposure to antigen (the foreign body). The four salient features of an acquired immunity are Antigen specificity, diversity of the immune system in recognition of molecule, the immunologic memory which stores the information of the type of antigen attacked and the type of immunity developed to the same and retrieve the information on the second attack by the same antigen and finally the ability to recognize the self and non-self antigens.

Acquired immunity is classified into active and passive acquired immunity each again falls into two categories called as the natural and artificial immunity. Active acquired immunity is the immunity developed by the body in response to an antigen, which once developed seems to exist in the individual forever and also it is stored in the memory of the immune system and exhibited on attack by the same antigen for the second time. Natural active immunity is the immunity developed naturally in response to any infection whereas artificial active immunity is induced in an individual with the help of vaccines.

Passive immunity is the induction of immunity in the immune deficient individual by introducing antibodies or immune cells as such to them. Unlike active immunity, passive immunity exists only for a short term and no immunological memory takes place. The transfer of immune particles from mother to fetus via placenta or breast milk is an example of natural passive immunity whereas the introduction of processed serum rich in antibodies to an immune deficient host is artificial passive immunity.

Though the immune system of our body challenges most of the pathogens, the evolution of new viruses challenges the immune system which can be explained by an example of the action of HIV on the immune system.