Innate Immunity: Non-specific Defenses
The innate immune response is rapid and antigen-independent
Innate immunity, the body’s first line of defense, is non-specific and activates independently of antigens, allowing for the rapid identification and elimination of foreign threats.1 Numerous cell types are involved with the innate immune response, including macrophages, neutrophils, dendritic cells, mast cells, basophils, eosinophils, natural killer (NK) cells, and lymphocytes (T cells).1 The primary effector cells of the innate immune response, NK cells, continually scan the body for abnormal cells to attack.1-3
NK cells do not rely on antigens to identify nonself invaders.1 Instead, NK cells express receptors that interact with activating and inhibitory signals from normal and abnormal cells. The balance of these signals determines NK-cell behavior.4 By engaging inhibitory receptors on NK cells, normal cells are able to identify themselves as self and protect against immune attack.5 In contrast, tumor cells express ligands that are not typical of normal cells.2,6 When the activating signals of tumor ligands prevail, they stimulate NK-cell antitumor immunity.5,7
NK cells are part of the first line of defense against cancer
Upon recognition of a tumor cell through engagement of an activating receptor, NK cells proliferate and rapidly kill their target.3,8 Following tumor cell death, the NK cells move on in search of other targets.3 In death, tumor cells can release tumor antigens and other factors.9-11
APCs act as primary messengers between innate and adaptive immunity
Tumor cell death allows the release of factors, such as adenosine triphosphate (ATP) and tumor DNA, that can cause the activation of APCs, including dendritic cells.11,12 APCs act as messengers between the innate and adaptive immune response.1
Inflammatory signals such as ATP can trigger the formation of an inflammasome within the APC.11,12 Inflammasomes are protein complexes that initiate an inflammatory immune response by converting proinflammatory cytokines from a dormant to an active state.13 Once activated, these cytokines are released by APCs to increase the antitumor activity of NK and T cells.12-14
Tumor DNA can be detected by sensors within the APC.15 These sensors stimulate the APC to engulf proteins released during tumor cell death and process them into antigens.11,16,17 One of the primary functions of APCs is to present these antigens to inactive T cells.17 The first presentation of an antigen to an inactive T cell leads to its activation and proliferation. This process, known as T-cell priming, initiates the adaptive immune response.17
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