The word immune literally means “free from burden. “Used to a general sense,immunity refers to the ability of an organism to regonize and defend itself against infectious agents. Susceptibility,the opposite of immunity,is the vulnerability of the host to harm by infectious agents. Host organisms have many general defenses against invading infectious organisms Immunity produced bu such defenses is called innate immunity. In contrast,adaptive immunity is the ability of a host to mount a defense against particular infectious agents by physiological responses specific to that infectious agent.

Innate immunity,also called genetic immunity,exists because genetically determined characteristics. One kind of innate immunity is species immunity,which is common to all members of a species. For example,all humans have immunity to many infectious agents that cause disease in pet and domestic animals,and animals have similar immunity to some human diseases. Innate immunity also includesthe ability of an organisma to recognize pathogens. (Microbilogy,7th edition by JACQUELYN G. BLACK) Innate immunity,the body’s built-in ability to regonize and destroy pathogens or their products.

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Innate immunity is largelly a function of phagocytes,cells that can engulf,kill and digest most pathogens. Innate responses recognize a variety of common structural features found on foregion molecules and pathogens,and nearlly all phagocytes have the ability to interact with most pathogens. Interaction in pathogens stimulate large number of phagocytes to activate a number of genes leading to the transcription,translation and expression of proteins that destroy the pathogens. Because many phagocytes are involved,innate immunity developes immediately upon contact with pathogens.

Unfortunately,innate immune responses are not always effective and dangerous infections sometimes occurs. However the phagocytes can activate another defense mechisms called adaptive immunity or specific immunity to deal with this infections. Adaptive immunity is acquired ability to recognize and destroy the pathogens or its products and is activated by exposure of the immune system to the pathogens. These molecules are called them antigens. Certain phagocytes pass pathogen molecules to lymphocytes.

Where the pathogen molecules interact with specific receptors on the lymphocytes. The pathogen molecule-lymphocyte receptor interaction activate the lymphocytes to transcribe or translate genes that produce pathogens specific protein. These protein then interact with the individual pathogen,making it for destruction. A protective adaptive response usually takes several days to developes because only few lymphocytes are available to interact with each pathogens;the strength of the adaptive response increase as the pathogen-reactive lymphocyte grow and their numbers multiply.

The cells involved in the innate and adaptive host responses to pathogens and other foreign substance. Beign with the cells and organ common to the entire immune system and then consider the cells and mechanisms involved in innate immunity. (Microbiology An Intriduction,BARRY L. BAZING) In (Microbiology 7th edition,Jacquelyn G. Black),In contract with innate immunity also called acquired immunity is immunity obtained in some manner other than by heredity. It can be naturally acquired or artificially acquired. Naturaly acquired adaptive immunity is most often obtained by having a specific disease.

During the course of the disease,the immune system responds to molecules called antigens on invading infectious agents. It activates cells called T cells,produces molecules called antibodies,and initiates other specific defenses that protect against future invasions by the same agent. Immunity also can be naturally acquired from antibodies transferred to a fetus across the placenta or to and infants in colostrum and breast milk. Colostrum is the fluid secreted by the mammary glands after childbirth. Althrought deficient in many nutrients found in milk

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