MD-2 is a novel molecule essential for lipopolysaccharide (LPS) signaling through Toll-like receptor 4 (TLR4). TLR4, a mammalian homologue of the *Drosophila* Toll receptor, is involved in innate immune responses against pathogens. However, transfection of TLR4 alone does not confer LPS responsiveness, suggesting the need for an additional molecule. MD-2, which physically associates with TLR4 on the cell surface, is shown to be necessary for LPS signaling. MD-2 acts as a link between TLR4 and LPS signaling, enabling TLR4 to respond to LPS. This discovery has significant implications for understanding host defense mechanisms and pathophysiology. MD-2 is structurally similar to MD-1 and shares several cysteine residues. It is expressed in various human and mouse cell lines and tissues. MD-2 is physically associated with TLR4, and this interaction is confirmed through immunoprecipitation experiments. MD-2 enhances TLR4-dependent activation of NF-κB, a key transcription factor in inflammatory responses. When expressed alongside TLR4, MD-2 enables LPS responsiveness, as demonstrated by increased NF-κB activity in response to LPS or lipid A. The study also shows that TLR4 and MD-2 form a complex on the cell surface, which is crucial for detecting bacterial endotoxins. MD-2 may also contribute to the broader recognition of pathogens by other TLR family members. The identification of MD-2 as a critical component of the TLR4 signaling pathway provides new insights into the mechanisms of innate immunity and may lead to new therapeutic strategies for infectious diseases and endotoxin shock.MD-2 is a novel molecule essential for lipopolysaccharide (LPS) signaling through Toll-like receptor 4 (TLR4). TLR4, a mammalian homologue of the *Drosophila* Toll receptor, is involved in innate immune responses against pathogens. However, transfection of TLR4 alone does not confer LPS responsiveness, suggesting the need for an additional molecule. MD-2, which physically associates with TLR4 on the cell surface, is shown to be necessary for LPS signaling. MD-2 acts as a link between TLR4 and LPS signaling, enabling TLR4 to respond to LPS. This discovery has significant implications for understanding host defense mechanisms and pathophysiology. MD-2 is structurally similar to MD-1 and shares several cysteine residues. It is expressed in various human and mouse cell lines and tissues. MD-2 is physically associated with TLR4, and this interaction is confirmed through immunoprecipitation experiments. MD-2 enhances TLR4-dependent activation of NF-κB, a key transcription factor in inflammatory responses. When expressed alongside TLR4, MD-2 enables LPS responsiveness, as demonstrated by increased NF-κB activity in response to LPS or lipid A. The study also shows that TLR4 and MD-2 form a complex on the cell surface, which is crucial for detecting bacterial endotoxins. MD-2 may also contribute to the broader recognition of pathogens by other TLR family members. The identification of MD-2 as a critical component of the TLR4 signaling pathway provides new insights into the mechanisms of innate immunity and may lead to new therapeutic strategies for infectious diseases and endotoxin shock.