There are many components of the innate immune system. A partial list and a brief description of their function are listed in Tables 4–6. The following pages will focus on just a few of these that might be less familiar to the reader: NK cells, Complement, and Toll-like receptors.
Table 4. Selected Cells of the Innate Immune System and their Function.| Cells |
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| Neutrophils | Phagocytic cells which quickly enter the site of infection in large numbers.
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| Macrophages | Phagocytic cells, which reside in most tissues, have multiple receptors and can present antigens to T lymph, release cytokines, and recruit other immune cells. |
| NK cells | Large non-specific granular cytotoxic lymphocytes recognize intracellular infections and tumor cells and then kill them by inducing apoptosis.
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Table 5. Selected Proteins and Peptides of the Innate Immune System and their Function.| Proteins/Peptides |
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Complement
| A series of innate plasma proteins become activated in a cascade fashion; these activated proteins can then enhance (“opsonize”) the pathogen for phagocytosis or directly perforate the pathogen cell membrane. |
| Cytokines | A large variety of proteins is secreted by cells that attach to receptors on other cells, leading to a specific activity in those other cells. For instance, cytokines will determine how a CD4 T cell will differentiate. |
Antimicrobial peptides
| Soluble effector molecules, also known as “defensins,” protect mucosal surfaces by directly penetrating microbial membranes, which can lead to the destruction of many types of pathogens.
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Other plasma proteins
| Many proteins whose function is not directly part of the immune system but play a role in defense; examples include fibrin, platelet granules, kinins, protease inhibitors, and others. |
Table 6. Selected Receptors of the Innate Immune System and their Function.| Receptors |
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Toll-like receptors (TLRs)
| A family of receptors found on macrophages, dendritic cells, neutrophils, and others that sense types of pathogen molecules called PAMPs - pathogen-associated molecular patterns - (such as lipopolysaccharide from bacterial walls) and lead to the activation of that cell. These activities are also crucial for initiating the adaptive immune response.
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Innate receptors
| A wide variety of receptors, such as complement receptors, lectins, and scavenger receptors (which can recognize pathogens’ nucleic acids and lipoteichoic acid.)
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