The collective interaction between cells is, in part, mediated by different 
families of adhesion molecules. Intercellular adhesion molecules (ICAMs) are 
structurally related members of the immunoglobulin supergene family and are 
ligands for the beta2 integrin molecules present on leukocytes. Of the five 
ICAMs identified, ICAM-1 is the most extensively studied. Although ICAM-1 is 
expressed constitutively at low levels on endothelial cells and on some 
lymphocytes and monocytes, its expression can be significantly increased in the 
presence of cytokines (TNFalpha, IL-1, IFNgamma) and reactive oxygen species. 
Depending upon cell type, ICAM-1 participates in trafficking of inflammatory 
cells, in cell:cell interactions during antigen presentation, in microbial 
pathogenesis, and in signal transduction through outside-in signaling events. 
Again, depending upon cell type examined, ICAM-1 engagement has been documented 
to activate specific kinases through phosphorylation, resulting in transcription 
factor activation and increased cytokine production, increased cell membrane 
protein expression, reactive oxygen species production, and cell proliferation.