What is Pathology - What causes leukocytes to become activated?

What is Pathology - What causes leukocytes to become activated?
​Leukocyte activation is caused by cell surface events that stimulate plasma membrane receptors. This usually happens after leukocytes attach to endothelial cells or interleukins bind to receptors on the plasma membrane of leukocytes. Signals from cell surface receptors can activate phospholipase C, which produces lipid-derived messengers including diacylglycerol (DAG) and inositol triphosphate (ITP) (IP3). As a result of the metabolic changes, the concentration of cytosolic calcium ions rises. Calcium plays a critical role in the activation of various intracellular processes that are critical for leukocyte function.