Immiscible immunity

Oil and water do not mix. This simple fact drives the formation of the lipid-based boundaries of cells, nuclei, and most organelles that make life possible. But a simpler structure, the lipid droplet (LD), coalesces in the cell cytoplasm and comprises a core of neutral fatty acids [mostly triacylglycerides (TAGs) and sterol esters] surrounded by polar lipids (phospholipids and sterols) and associated proteins. An LD is, in essence, a drop of oil, but it has properties that many proponents suggest give it organelle status. As a rich energy reserve, LDs fuel β-oxidation in m itochondria and often physically associate with these organelles. It follows, however, that invading microorganisms similarly exploit LDs as an energy source and, therefore, that cells might “arm” LDs with innate host defense mechanisms. On page 309 of this issue, Bosch et al. (1) explore the relations between the antimicrobial activities of LDs and their function as a fuel depot for mitochondria.