Phylogenetic development. Evolutionarily, the innate immune system evolved or developed before the adaptive immune response and appears to be inherent in all multicellular organisms. One feature that differentiates the innate and adaptive immune systems is the possession of germ-line-encoded receptors found within the innate immune system. These germ-line-encoded receptors mediate innate immune recognition.
These receptors are predetermined genetically and thus have evolved through natural selection to possess specificity against infectious microorganisms. The immune system in higher animals and humans evolved with increasing complexity in terms of its specific antibody and cell-mediated immune response capacity, i.e., the adaptive immune response. Despite the complexity of the human immune system, host defense is still highly dependent on surface barriers and phagocytic mechanisms. The absence of phagocytic functions (as in severe neutropenia or in neutrophil function disorders, such as chronic granulomatous disease) or the loss of physical barriers (as with extensive cutaneous burns) can present a risk of fulminant, life-threatening invasion by microorganisms that normally are not pathogenic.
These receptors are predetermined genetically and thus have evolved through natural selection to possess specificity against infectious microorganisms. The immune system in higher animals and humans evolved with increasing complexity in terms of its specific antibody and cell-mediated immune response capacity, i.e., the adaptive immune response. Despite the complexity of the human immune system, host defense is still highly dependent on surface barriers and phagocytic mechanisms. The absence of phagocytic functions (as in severe neutropenia or in neutrophil function disorders, such as chronic granulomatous disease) or the loss of physical barriers (as with extensive cutaneous burns) can present a risk of fulminant, life-threatening invasion by microorganisms that normally are not pathogenic.
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