Types of immunity : Based on the nature of response, immunity is mainly of two-types, namely (i) Innate immunity and (ii) Acquired immunity .
i) Innate Immunity (Innate - inborm or present at the time of birth ) : The inborn resistance of diseases, possessed by all the living organisms is called innate immunity . It is a non-specific type of defence and does not depend on prior contact with the micro - organisms . This is executed by providing different types of barriers like : -
a) Physical barriers : Skin and mucous membranes are the main physical barriers . Skin prevents the entry of micro - organisms whereas the mucus membranes help in trapping the microbes entering our body .
b) Physiological barriers : Secreation of the body like HCl in the stomach, saliva in the mouth, tears from the eyes are the main physiological barriers against microbes .
c) Cellular barriers : Certain types of cells like polymorphonuclear leukocytes (PMN- neutrophils ) , monocytes and natural killer cells in the blood as well a macrophages in the tissues are the main cellular barriers . They phagocytose and destroy the microbes .
d) Cytokine barriers : The cytokines secreted by the immune cells are involved in differentiation of the cells of immune system and protect the non-infected cells from further infection .
ii) Acquired Immunity or Adaptive immunity : The immunological resistance developed by an individual throughout the life after the birth is known as acquired immunity or adaptive immunity . It is pathogen specific and depends on prior contact with the infections micro - organisms . Hence it is characterised by immunological memory . It varies from person to person . It is again of two types namely (A) Active acquired immunity and (B) Passive acquired immunity .
A) Active acquired immunity : The immunological resistance developed by the organisms through the production of antibodies in their body, is called active immunity . It is a lifetime - immunity (long lasting immunity ) . But it is slow and takes time to show its fully effective response . It is again of two types, namely (a) Natural active acquired immunity and (b) Artifical active acquired immunity .
a) Natural active immunity : The resistance developed by an individual in response to natural infection, from which a person recovers is called natural active acquired immunity . e.g. the lifetime immunity acquired by an individual after recovering from infections such as smallpox, chickenpox, etc .
b) Artifical active immunity : The immunity developed by an individual due to the inoculation of weakened antigens into the body is called artifical active acquired immunity . e.g., immunity that develops due to vaccination .
B) Passive acquired immunity : The immunological resistance developed by an organism due to the transfer of ready - made (preformed) antibodies is called passive acquired immunity . It is again of two types, namely (a) Natural passive immunity and (b) Artifical passive immunity .
i) Natural passive Immunity : If the preformed antibodies are transferred from mother to child, it is called natural passive acquired immunity e.g., Transfer of antibodies from mother to foetus across the placenta or from mother to child through colostrum .
ii) Artifical passive immunity : If the pre formed antibodies are transferred from an immunised donor to a non-immunised individual, it is called artifical passive acquired immunity . e.g., Injection of anti-tetanus serum (ATS), anti-rabies serum and serum containing antivenin against the venom of a snake, etc. These antibodies are generally produced in the body of an immunised horse or sheep .
Based on the types of responses evoked . immunity is of two types, namely . (i) Humoral immunity and (ii) Cell mediated immunity .
i) Humoral immunity (HI) : The immunity mediated by the antibodies that are released into the fluids of the body (humors) such as plasma, lymph, etc., is called humoral immunity . It is due to the interaction of B - cells with with free antigens .
ii) Cell Mediated Immunity (CMI) : The immunity mediated by the activated T-cells, natural killer cells, etc., is known as cell mediated immunity . It is effective against both exogenous and endogenous antigens . It does not involve antibodies .
