Immune cells are most often portrayed as patrolling soldiers looking for invaders. However, many immune cells do not move. They hunkered down at sites where the body interfaces with the environment. So let's see which cells move and which cells do not. Tissues at the interface with the outside environment such as lungs, guts, and skin need more than circulating cells to efficiently monitor danger. Many specialized innate and adaptive like immune cells reside permanently in these tissues, like soldiers in a fort, and self-renew as needed. Most resident cells are located within the epithelium or within the underlying tissues. For example, in the skin, the epithelial layer is colonized by intraepithelial lymphocytes and specialized dendritic cells called Langerhand cells. The tissue underlying the epithelial layer is called the dermis. The dermis is populated by resident dendritic cells, innate like cells, macrophages, and mass cells. The role of these resident cells is to immediately deal with local threats, and, if overwhelmed, call for reinforcement. There are two types of circulating cells, cells that sample antigens all over the body and cells that act as first responders. Most responders are mostly innate immune cells like neutrophils, basophils, monocytes, and eocinophils. They move through the vasculature and largely migrate into a tissue only in response to inflammatory signals. T lymphocytes and natural killer cells are cells that move throughout the body to sample antigens. They patrol the periphery using blood vessels to enter tissues, sample antigens, and rejoin the vasculature when no antigens have been met. T lymphocytes can also use the lymphatic system to move around the body. In the lymphatic system, T lymphocytes will migrate from one draining lymph node to the next, sampling antigens presented by antigen presenting cells as they go. This allows efficient monitoring of antigens and increases the chance for a T lymphocyte to recognize its specific antigen. In short, depending on their function, immune cells either patrol or hold down the fort. Their protective effort is integrated whereby local responders are specialized in monitoring and responding to the health of a particular environment and can call on circulating cells to help if needed. Ultimately, cells that move and cells that hunker down work together to keep invaders at bay.