A single mast cell enters conjunctival tissue in the eye in response to an inflammatory agent or pathogen. Mast cells are among the first cells of the immune system to react to the presence of an invader, facilitating movement of leukocytes and other immune cells toward the site of infection through the release of histamines (red dots), which act as chemical messengers.
Image courtesy of Donald Pottle, The Schepens Eye Research Institute, Massachusetts.