The so called sentinel cells are located in our skin to detect invasions by e.g. microbes and viruses in terms of the innate immune response. Macrophages and dendritic cells as well as mast cells belong to the class of antigen representing sentinel cells. When an infection is caused by invaders entering our tissue through a damaged epidermis (wound), the sentinel cells respond by releasing cytokines (there are about 50 different cytokines e.g. TNF and Interleukins [IL-1,IL-2, ... , IL-35]). Sentinel cells do not recognise molecules that allows them to classify specific cell types but rather broad classes of invaders such as for exapmle viruses or a bacterial cells (as opposed to the adaptive immune response that is more specific).
Cytokines cause endothelial cells in the blood vessle to produce adhesion molecules (E-selectin, P-selectin) and lead to vasodilation (wideing of bood vessles). This results in extravasion of rolling leukocytes into the tissue and allows antibodies to enter the infection site. Only specific leukocytes are selected for extravasion by so called chemokines that are specifically recognised by chemokine receptors of the leukocyte. The recruited leukocytes then help to fight the infection. This process is known as inflammation and is accompanied by the cardianl signs: Dolor (pain), Calor (heat), Rubor (redness), Tumor (swelling), Functio laesa (loss of function).