The function of ICAM1 (intercellular adhesion molecule 1, Ensembl gene identifier ENSG00000090339) is as follows. Cell adhesion molecule that functions as a receptor ligand of the signaling receptor ITGAL:ITGB2/LFA-1 (lymphocyte-function associated (LFA) molecule 1) ensuring leukocyte cell-cell adhesion, by providing a calibrated system to namely adjust T-cell killing to the antigen stimulation strength (PubMed:3086451, PubMed:3340213, PubMed:38195629). Also functions as a ligand receptor of the signaling receptor ITGAM:ITGB2/MAC-1 ensuring adhesion between stimulated neutrophils and stimulated endothelial cells (PubMed:1980124). During leukocyte trans-endothelial migration, ICAM1 engagement promotes the assembly of endothelial apical cups through ARHGEF26/SGEF and RHOG activation (PubMed:17875742). Promotes cell aggregation in epithelial cells through interaction with MUC1 (PubMed:11173916). {ECO:0000269|PubMed:11173916, ECO:0000269|PubMed:17875742, ECO:0000269|PubMed:1980124, ECO:0000269|PubMed:3086451, ECO:0000269|PubMed:3340213, ECO:0000269|PubMed:38195629}. (Microbial infection) Acts as a receptor for major receptor group rhinovirus A-B capsid proteins. {ECO:0000269|PubMed:1968231, ECO:0000269|PubMed:2538243}. (Microbial infection) Acts as a receptor for Coxsackievirus A21 capsid proteins. {ECO:0000269|PubMed:11160747, ECO:0000269|PubMed:16004874, ECO:0000269|PubMed:9539703}. (Microbial infection) Upon Kaposi's sarcoma-associated herpesvirus/HHV-8 infection, is degraded by viral E3 ubiquitin ligase MIR2, presumably to prevent lysis of infected cells by cytotoxic T- lymphocytes and NK cell. .