The monoclonal antibody 26-5 recognizes lipid A. The lipid A domain of lipopolysaccharide (LPS) is aunique, glucosamine-based phospholipid that makes up the outer monolayer of the outer membrane ofmost gram-negative bacteria.Lipid A is responsible for the endotoxic activities of LPS, and a heteropolysaccharide. Theheteropolysaccharide generally consists of an oligosaccharide termed the core region. This core regionis in some bacterial families composed of repeating oligosaccharide units, the so-called O chain. Thelipid A moiety of LPS is detected by the TLR4/MD2 receptor of the mammalian innate immune system.Picomolar levels of lipid A induce macrophages to synthesize potent mediators of inflammation, such asTNF-α and IL-1β. Furthermore, lipid A activates the production of costimulatory molecules required foradaptive immunity and stimulates tissue factor production in mononuclear and endothelial cells. Allthese events are desirable for clearing local infections.