Staphylococcal enterotoxin that activates the host immune system by binding as unprocessed molecules to major histocompatibility (MHC) complex class II and T-cell receptor (TCR) molecules. In turn, this ternary complex activates a large number of T-lymphocytes initiating a systemic release of pro-inflammatory cytokines (PubMed:2210803, PubMed:8039910). Inhibits SEC1-mediated T-cell activation in the absence of MHC class II by competing with SEC1 for binding to the host TCR (PubMed:9498747). Causes also the intoxication staphylococcal food poisoning syndrome (By similarity). Interacts with host MHC class II molecules composed of alpha/HLA-DRA and beta/HLA-DRB1 chains. Belongs to the staphylococcal/streptococcal toxin family.