Immunology | Immune system| Immunity

               The field of immunology is concerned mostly with how innate and adaptive mechanisms collaborate to protect vertebrates from infection.Although many cellular and molecular actors have important roles,antibodies and lymphocytes are considered to be the principal players .Yet despite their prominence in vertebrate immune systems,it would be a mistake to conclude that these extraordinary molecules and versatile cells are essential for immunity.In fact, determinants search for antibodies,T cells,and B cells in organisms of the non vertebrate phyla has failed to find them.The interior spaces of organisms as diverse as fruit flies,cockroaches,and plants do not contain unchecked microbial populations,however,which implies that some sort of immunity exists in most,possibly all,multicellular organisms,including those with no components of adaptive immunity.
            A major difference between adaptive and innate immunity is the rapidity of the innate immune response,which utilizes a pre-existing but limited repertoire of responding components.Adaptive immunity compensates for its slower components.Adaptive immunity compensates for its slower onset by its ability to recognize a much wider repertoire of foreign substances,and also by its ability to improve during a response,whereas innate immunity remains constant.It may also be noted that secondary adaptive responses are considerably faster than primary responses.Principle characteristics of the innate and adaptive immune systems.
            Insects and plants provide particularly clear and dramatic examples of innate immunity that is not based on lymphocytes.The invasion of the interior body cavity of the fruit fly ,Drosophila melanogaster,by bacteria or molds triggers the synthesis of small peptides that have strong antibacterial or anti-fungal activity.The effectiveness of these antimicrobial peptides is demonstrated by the fate of mutants that are unable to produce them.For example,a fungal infection overwhelms a mutant fruit fly that is unable to trigger the synthesis of drosmycin, an antifungal peptide.Further evidence for immunity in the fruit fly is given by the recent findings that cell receptors recognizing various clases of microbial molecules were first found in Drosophila.
                 Plants respond to infection by producing a wide variety of antimicrobial proteins and peptides,as well as small nonpeptide organic molecules that have antibiotic activity.Among these agents are enzymes that digest microbial cell walls,peptides and a protein that damages microbial membranes,and the small organic molecules phytoalexins.The importance of the phytoalexinsis shown by the fact that mutations that alter their biosynthetic pathways result in loss of resistance to many plant pathogens goes beyond this chemical assault to include in the infected area  by strengthening the walls of surrounding cells.