Acquired or Adaptive Immunity

Adaptive immunity provides a defence against some of the pathogens that avoid the innate immune system and can mount an attack against the evolving and ever changing characteristics of disease-causing organisms, e.g., different strains of bacteria and viruses, such as those that cause influenza. 

The cells that are the principal agents of adaptive or acquired immunity are from the lymphoid lineage of stem cells. Similar to the situation with myeloid cells, mutations in lymphoid precursor cells give rise to the lymphocytic leukemias. 

In summary, acquired immunity is a lymphocyte-dependent (B and T cell) process by which molecular properties of infectious agents are recognized, anti-infectious 

NK cells, monocytes, neutrophils, and macrophages are involved in IgG-mediated ADCC (see below) eosinophils are involved in IgE-mediated ADCC. 

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Rodent and human immunology has served to guide our understanding of the mammalian immune system, where the highly conserved innate immune system, and the more complex acquired or adaptive immune system, interact to protect the host from infection. 

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. ... 

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). 

Immune systems in animals and plants are quite different. There are two types of immune systems in animals (1) innate, so-called non-specific or passive immunity (2) adaptive, so-called specifically acquired , active, or cell-mediated immunity. Innate immunity is based on barriers to infectious agents and adaptive immunity is based on multiplicative and specific antibody release after contact with an antigen (infectious agent). The so-called memory cells in animals respond to secondary contact with an antigen. 

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]. 

The entire adaptive immune system was acquired by the immediate ancestors of the sharks (the extinct Placoderms), either by Transib retrotransposons, or by a herpesviral ancestor of the EBV lineage, or both (vide supra). Constituents of the adaptive immune system (MHC, Ragl/2, RSS, V(D)J) united first in the ancestral sharks the innate and adaptive immune systems were extensively reviewed [27, 147]. 

The immune system may be described as innate, passive or acquired (adaptive). Acquired immunity is further divided into humoral and ceU-mediated types. 

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. 

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