White Blood Cells
In some cases, cells from the blood need to pass through blood vessel walls in order to carry out their function. White blood cells, for example, are an important part of the immune system and travel throughout the circulatory system but may need to access other tissues in order to fight off infections or cellular damage. A report in the 2003 issue of "Biophysical Journal" explains that white blood cells are able to naturally pass through the walls of capillaries, which are small and thin blood vessels in the body. White blood cells pass through these vessels by binding to proteins found on the blood vessel walls. At first, this binding only causes the white blood cell to slow down and "roll" along the wall. Eventually (and possibly with the help of signals from red blood cells) the white blood cell is able to stick to the wall of the capillary and slip in between capillary cells, which allows it to escape the circulatory system.
VPF
Another way in which blood cells can pass through blood vessel walls is through the action of a chemical called vascular permeability factor, or vascular endothelial growth factor. An article published in the November 2002 issue of the "Journal of Clinical Oncology" explains that this hormone triggers a series of chemical messages that cause blood vessels to be leaky. When capillaries become leaky, the space in between blood vessel cells expands, which allows fluids, dissolved substances, and blood cells to pass through the gaps.
Endothelial Dysfunction
A more serious cause of blood cells escaping from blood vessels is known as endothelial dysfunction. The endothelium is the name for the cells that form the inner lining of blood vessels; it is their job to maintain the integrity of the blood vessel walls. A 2009 article about pulmonary vascular permeability published in "Critical Care" explains that some diseases, including bacterial sepsis that results from bacterial growth in the blood, cause endothelial dysfunction. This means that the endothelial cells cease to be an effective barrier between the inside and the outside of the blood vessel. When blood cells hemorrhage, it allows bacteria to access tissues outside of the blood vessels.
