Which substance is primarily involved in chemotaxis, directing immune cells to the site of injury?

• A. Peritoneum
• B. Bradykinin
• C. Exudate
• D. Cytokines

Answer: (D)

Chemotaxis is the directed movement of leukocytes toward higher concentrations of chemical signals released at the injury site. The main signals that guide this migration are chemokines, a subset of cytokines. These small signaling proteins are produced by activated macrophages, endothelial cells, and other cells in the inflamed area and establish a gradient that neutrophils and other leukocytes follow from the bloodstream into the tissue. Receptors on the leukocytes detect these chemokines, triggering adhesion to the endothelium, transmigration, and movement toward the source of the signal. IL-8 is a classic example of a chemokine that recruits neutrophils. Bradykinin mainly causes pain and increases vascular permeability, not the targeted chemotactic pull, while exudate and peritoneum describe tissue or fluid aspects rather than signaling molecules.