Although endotoxin is found in high concentrations in nature, Liu says, it's also found in urban settings because bacteria are everywhere. Moreover, researchers had previously demonstrated that endotoxin can prime the immune system to respond to allergy-causing substances. To understand how this occurs, consider the workings of the immune system. More than just the sum of its parts, the immune system is a highly coordinated network of millions of white blood cells that attack and kill invading microbes. At the top of the heap are T cells, specialized white blood cells that are capable of distinguishing self from non-self and coordinating the human immune response.
T cells come in many varieties.
Some act as dispatchers, squirting out potent signaling molecules that can activate and deactivate other immune cells. Some T cells are capable of killing virus-infected cells. Without T cells, no one could survive for very long. Witness the effect of AIDS, caused by a virus that kills T cells. Allergy might best be thought of as a case of arrested T-cell development.