A number of mechanisms operate to protect the body from attack by foreign bodies and toxins, including infections by bacteria. These mechanisms can be classified as nonspecific mechanisms. Mechanisms specific to invading foreign proteins called antigens, which stimulate the immune system. This chapter covers the sulcular fluid, anatomy of gingival crevice or sulcus, significance of gingival sulcus and fluid, significance of gingival vasculature and crevicular fluid, permeability of sulcular and junctional epithelium, methods of collection, composition of gingival crevicular fluid, and clinical significance. The structure and function of the gingival marginal area are now known with more precision. In a perfectly sound histologically normal gingiva, a few polymorphonuclear neutrophils can be seen migrating through the junctional epithelium, while very little or no gingival fluid can be collected.