Adaptive/acquired immune response

As indicated, one of the primary responsibilities of the immune system is to protect the body from bacteria, viruses, and other foreign pathogens. The immune response consists of two primary components innate and adaptive (acquired) immunity.18,23 Innate immuni-... 

Complement acts to kill bacterial cells that are missed by the neutrophils and the macrophages. There arc actually two separate complement pathways. One. the classical pathway, operates in the adaptive or acquired immune response. The elas.sieal pathway has an absolute requirement fur an Ab-an-tigen complex as a trigger. The other, the alieriimive pathway. requires no Ah or antigen to initiate and is operative in innate immunity. Both pathways operate in a tightly regulated cascade fashion. The proteins normally circulate as inactive proenzymes. When the pathways are activated, the product of each step activates the subsequent step. 

Immune responses are the result of an effective collaboration between innate (natural and relatively nonspecific) and acquired (adaptive, and extremely specific) com-... 

The innate and adaptive branches of the immune response are both needed for optimal immune function, and the two interact extensively.18,23 The adaptive response s ability to recognize and deal with foreign pathogens likewise involves an incredibly complex interaction between various cellular and chemical (humoral) components.23 48 51 A detailed description of the intricacies of how these components work together is beyond the scope of this chapter. Many aspects of the immune response are still being investigated. An overview of key cellular and humoral elements that mediate acquired immunity is illustrated in Figure 37-1, and these elements are described briefly below. 

These mature, but naive T cells exit peripheral blood and seed lymphoid organs in T-cell specific zones to be acquired by adaptive immune responses for elimination of infected or tumor cells, support for humoral immune responses, formation of immunologic memory, prevention of excessive tissue damage, and facilitation of tissue regeneration (see Chapter 12). 

The immune system plays a central role in the pathogenesis of prion infection. In the acquired human prion diseases, it is the initial site of agent replication prior to entry into the nervous system but the immune system does not appear to be essential in the pathogenesis of the familial and sporadic human prion diseases. Followdng peripheral exposure to the prion agent, the immune system can amplify prion infection but, interestingly, the adaptive immune response does not play a role in the clearance of the prion agent. 

Besides the innate immune system that has always been considered the main factor responsible for the biomaterial-associated host responses, the acquired immune system may also start to reveal an important role in these responses. Interestingly, these developments coincide with the emergence of new concepts in macrophage be-haviom, where it is recognized that these cells that orchestrate the tissue response can adapt to different phenotypes ranging from the proinflammatory Ml (classically activated) to the prohealing M2 (alternatively activated), most likely in a continuous spectrum of activities (Scislowska-Czamecka et al., 2012 Mosser and Edwards, 2008). 

Fish represent the earliest class of vertebrates in which both innate and acquired, or adaptive, immune mechanisms are present. The innate immune system appears to play a central role in the response to infections in fish, whereas in mammals the adaptive immune system is more significant. The intrinsic inefficiency of the adaptive immune response in fish is due to its evolutionary status - it only possesses IgM-like responses - and, moreover, due to environmental constraints such as temperature, because of the poi-kilothermic nature of fish. These factors result in a limited antibody repertoire, poor affinity maturation and memory, slow lymphocyte proliferation, and a short-lived secondary response. ... 

It remains unexplained why the teleost Atlantic cod (Gadus morhua) failed to acquire MHC class II system with its invariant chain, and reactive CD4 T cells. Was it never acquired in the cod lineage, or was it lost to deletion, or is the cod operating an unusually effective innate immune system Indeed, it appears fliat the cod expanded its entire innate and the MHC class 1 adaptive immune system. There is adequate response to endotoxin, and to bacterial (Aeromonas salmonicida Vibrio anguillarum) and viral (pancreas necrosis virus) pathogens [987-989]. 

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