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[edit] Characteristics

Natural killer cells can be distinguished from other lymphocytes by their large size as well as the presence of large numbers of granules in the cytoplasm. They are further distinguished by the cell surface expression of the NK1.1 molecule and the lack of T-cell receptor expression.

[edit] Activation

Given their strong cytolytic activity and the potential for auto-reactivty, Natural Killer cell activity is tightly regulated. Natural Killer cells must receive an activating signal which can come in a variety of forms, the most important of which are listed below.

• Cytokines

The cytokines IFNα/β plays a crucial role in NK-cell activation. As these are stress-molecules, released by cells upon viral infection, they serve to signal to the NK-cell the presence of viral pathogens. The ubiquitous activator IL2 as well as IFNy have been demonstrated to also have the capability of activating NK-cells.

• FcR receptor

NK-cells, along with macrophages and several other cell types, express the FcR molecule, an activating receptor which binds the Fc portion of antibodies. This allows Natural Killer cells to target cells against which a humoural response has been mobilized and lyse cells through Antibody-dependant cell cytotoxicity.

• Activating and inhibitory receptors

Aside from the FcR receptor, Natural Killer cells express a variety of receptors which serve to either activate or suppress cytolytic activity. These bind to various ligands on target cells, both endogenous and exogenous, and have an important role in regulating the NK-cell response.

Viruses: LCMV, herpesvirus, influenze, coxsackie. HIV. Response can take several hours, compared to days for t and b cell adapive responses.

Effector Function Cytotoxicity, Cytokine production (IFNy and TNF) especially IFNy serves to activate surrounding macrophages, induce iNOS, and block replication. (114) Chemokines