Using DNA microarray technology, researchers at the Whitehead Institute for Biomedical Research have discovered that a type of human immune cell, known as a dendritic cell, initiates an immune response that is tailor-made for specific infectious organisms. The researchers found that dendritic cells turn on different sets of genes, or a signature pattern of gene response, depending on whether the organism is a bacteria, virus, or fungus. This study shows that even at the earliest stages of infection, the human body knows the nature of the infectious organism, or pathogen, and responds with a specific type of immune response to eliminate the pathogen.