There are many components of the innate immune system. A partial list along with a brief description of their function is listed in Tables 4-6. The following pages will then focus on just a few of these which 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.
|Neutrophils||Phagocytic cells which quickly enter site of infection in large numbers|
|Macrophages||Phagocytic cells resident in most tissues; have multiple receptors and are capable of antigen presentation to T lymphs, release of cytokines, and recruitment of other immune cells|
|NK Cells||Large non-specific granular cytotoxic lymphocytes; recognize intracellular infections and tumor cells and then kill them by inducing apoptosis.|Table 5. Selected Proteins and Peptides of the Innate Immune System and their Function.
|Complement||A series of innate plasma proteins which become activated in a cascade fashion; these activated proteins then can enhance (“opsonize”) the pathogen for phagocytosis or directly perforate the pathogen cell membrane. |
|Cytokines||A large variety of proteins secreted by cells which attach to receptors on other cells and lead 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” that protect mucosal surfaces by directly penetrating microbial membranes which can lead to destruction of many types of pathogens.|
|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.
|Toll-like receptors (TLRs)||A family of receptors found on macrophages, dendritic cells, neutrophils, and others which 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 important for initiating the adaptive immune response.|
|Innate receptors||A wide variety of receptors such as complement receptors, lectins, and scavenger receptors (which can recognize pathogens’ nucleic acids and leipoteichoic acid.)|