The white blood cells' function is mainly to fight off external antigens. However, another one of its traits is its ability to bind to vascular endothelial cells with the help of L-selectin. What is the purpose of this, and how does this help us？
Fighting of harmful microbes (and not antigens per se, although specific antigenic recognition is important), involves a lot of processes which require several lignad-binding molecules and ligands on the WBC surface. While the whole array of these proteins and their functions is very complex, we can take the case of a neutrophil trying to defend the body against microbes in a local wound, the so-called acute local inflammation response.
1. Adhesion to the endothelium
Here is where several selectins play a role. The simple broad idea is that in presence of microorganisms, as sensed by the local immune cells (tissue macrophages, blood cells in case of a vessel breach etc), there is an upregulation of chemokines in the local environment which causes the endothelium to increase the expression of certain adhesion molecules (ligands with receptors on WBC and vice versa) as well as other hemodynamic changes causing the neutrophil to come in close contact with these in the vicinity of the wound and bind to the endothelium. This is ften subdivided into three processes, margination, rolling and adhesion. L-selectin is one of these adhesion mediators, along with other selectins and integrins.
2. Migration through the endothelium into the tissues (diapedesis)
3. Chemotaxis towards microbes
4. Action on the microbes
Points 2,3 and 4 have their own mediators (see CD31, chemokines - AA, C5a, leukotrienes).
Other sources: Wikipedia page