I refer you to the lyrics of the great scientific work/rap song “Ridin’ Dirty” by Chamillionaire*. “They see me rollin’”—selectins are weaker adhesion molecules that are expressed on blood vessel walls due to local inflammation caused by bacteria. These cause white blood cells to roll along the walls of the blood vessels (rather than floating) in the area of an infection. “They hatin’”—cytokines and chemokines, floating protein mediators of inflammation, cause white blood cells to be activated and get primed to eat and destroy bacteria “Patrollin’”—when they reach the area of the injury, where the maximum amount of chemokines are, the white cells use integrins, deep proteins under the endothelial cells, to pull their way out of the blood vessels. In the tissues they eat foreign material, continue to express inflammatory mediators, and many of them die, forming the gross white substance of puss. “Tryin’a catch me ridin’ dirty”—bacteria express foreign molecules on their cell surfaces such as lipopolysaccharide. These pathogen-associated molecular patterns identify them as foreign or dirty and tag them for endocytosis by neutrophils and macrophages. “Tryin’a catch me ridin’ dirty”—complement is a protein in the blood which can be activated through multiple inflammatory means and tags infectious organisms for destruction as well as kills infected cells. This is a second way for foreign material to be recognized.

Source: PhD in microbiology. Fan of rap.

Edit: Chamillionaire did Ridin' Dirty. I hereby rescind my claim to having a PhD.