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# Immunity Immunity is the resistance to the onset of a disease after infection by harmful germs. ## To define more elaborately Immunity is the "capacity of our body to deal with foreign substances, e.g., bacteria, viruses, toxins, etc. that enter our body and to render them harmless." or simply, "...

# Immunity Immunity is the resistance to the onset of a disease after infection by harmful germs. ## To define more elaborately Immunity is the "capacity of our body to deal with foreign substances, e.g., bacteria, viruses, toxins, etc. that enter our body and to render them harmless." or simply, "it provides resistance against disease-causing germs." ## Kinds of Immunity Immunity can be classified into two main categories - innate and acquired immunity and their different subcategories are as follows: ### 1. Innate Immunity Also called natural or native immunity. This immunity is by virtue of genetic constitutional make-up. It is there in the body without any external stimulation or previous infection. #### (a) Non-specific innate immunity A degree of natural resistance to all infections in general. For example, humans do not suffer from the plants' highly infectious diseases, or even certain diseases of animals. #### (b) Specific innate immunity This is a natural resistance to a particular kind of germ only. Some races or some individuals do not suffer from certain infectious diseases. For example, human beings are immune to a highly infectious disease of dogs known as 'Distemper, which kills about 50% of all infested dogs. ### 2. Acquired Immunity Resistance to a disease which an individual acquires during his life-time. It may be the result of: #### EITHER A previous infection (actively acquired immunity), e.g., a person having once suffered from "measles" will not normally suffer from it again. #### OR "Ready-made" antibodies supplied from outside (passively acquired immunity), e.g. a person bitten by a poisonous snake is injected with antivenin. Antivenin contains ready-made antibodies produced in the blood of a horse previously immunized by injecting a small dose of snake venom. ## Immunity ### 1. Innate (Inborn/natural) #### (a) Non-specific (For infections in general) #### (b) Specific (For particular infections) ### (a) Active (Produced by one's own body) #### (i) Natural (By previous infections) #### (ii) Artificial (by vaccinations) ### (b) Passive (Supplied from outside sources) #### (i) Natural (Antibodies in a foetus received from the mother's blood) #### (ii) Artificial (Readymade antibodies produced in other animals) ### 1. Acquired (During lifetime) ### (a) Active (Produced by one's own body) #### (i) Natural (Antibodies in a foetus received from the mother's blood) #### (ii) Artificial (by vaccinations) ### (b) Passive (Supplied from outside sources) #### (i) Natural (Antibodies in a foetus received from the mother's blood) #### (ii) Artificial (Readymade antibodies produced in other animals) ## Merits of Local Defence Systems 1. They start working instantaneously. 2. They are not dependent on previous exposure to infections. 3. They are effective against a wide range of potentially infectious agents. ## 17.2.2 Immune System The local defense systems are not effective in all cases and circumstances. Certain microbes (germs) or their poisonous secretions (toxins) do enter the deeper issues and various organ systems by their special mechanisms of entry or through any breaches in the protective barriers. At this stage, the blood and other body fluids start fighting against the germs or any other unacceptable, foreign substance. The body fluids contain special proteins called antibodies which react with the invading germs, and antitoxins which react with their poisons (toxins) to destroy them, and thus they provide protection against the diseases. ## WBCs get squeezed out of the walls of the blood ## capillaries (by diapedesis) and engulf the ## bacteria or the germs and destroy them ## (phagocytosis) (Figs. 17.1 and 17.2). Pus in a ## wound, for example, is a mixture of destroyed ## germs, killed WBCs and damaged tissue cells.