Nonspecific or natural resistance refers to barriers, secretions, and normal flora that make up our external defenses. Phagocytes and complement are also involved. Mechanical barriers are highly effective, and the skin (our largest organ) is highly suited to this protection; loss of a major part of the skin (secondary to burns, acids, etc) immediately exposes the host to marked susceptibility to infection. The mucosal lining of mouth and respiratory tract is another excellent defense mechanism. Yet, a defect in the mucosal lining of the respiratory tract, which occurs in cystic fibrosis, result in a hightened susceptibility to many infections. These are examples of a defect in the epithelium or epithelial lining. In general, however, it is the mobilization of the phagocytic cells such as monocytes/macrophages and polymorphonuclear leukocytes that ingest invading microorganisms and kill them.