Receptors that turn on and turn off natural killer cells

The receptors on natural killer cells for polymorphic major histocompatibility complex or human leukocyte antigen class I molecules have recently been cloned. Two structurally distinct receptor families have been identified from murine and human natural killer cells. These receptors are classified as members of the C-type lectin and immunoglobulin families. Initially, it appeared that murine natural killer cells express the C-type lectin receptors whereas human natural killer cells express immunoglobulin-like receptors. Recent data suggest that human natural killer cells can express both C-type lectin-like receptors and the immunoglobulin-like receptors. The interaction of natural killer-cell receptors with class I molecules was characterized to inhibit natural killer-cell activation; however, allelic forms of these natural killer cell receptors have recently been identified to also have activating properties. These discoveries show the presence of a diversity of receptors for major histocompatibility complex class I molecules on T and natural killer cells. In addition they illustrate the use of two unique strategies by the immune system for eliminating altered- or non-self cells: through the detection of foreign antigens and through the detection of absence of self antigens.