The respiratory bronchioles represent the transition from the conducting portion to the respiratory portion of the respiratory system. The narrow channels are usually less than 2 mm in diameter and they are lined by a simple cuboidal epithelium, consisting of ciliated cells and non-ciliated Clara cells, which are unique to bronchioles. In addition to being structurally diverse, Clara cells are also functionally variable. One major function they carry out is the synthesis and secretion of the material lining the bronchiolar lumen. This material includes glycosaminoglycans, proteins such as lysozymes, and conjugation of the secretory portion of IgA antibodies. These play an important defensive role, and they also contribute to the degradation of the mucus produced by the upper airways. The heterogeneous nature of the dense granules within the Clara cell's cytoplasm suggests that they may not all have a secretory function. Some of them may contain lysosomal enzymes, which carry out a digestive role, either in defense: Clara cells engulf airborne toxins and break them down via their cytochrome P-450 enzymes (particularly CYP4B1, which is only present in the clara cells) present in their smooth endoplasmic reticulum; or in the recycling of secretory products. Clara cells are mitotically active cells. They divide and differentiate to form both ciliated and non-ciliated epithelial cells.