The skin and hair are your pet's first line of defence against physical damage and microbial invasion. Fat underneath it offers insulation and further physical protection. Skin is durable, elastic and capable of excellent repair when damaged. It consists of an outer layer (epidermis) of sheets of flat scaly cells. These are shed through natural wear and tear (and some of us humans are allergic to a protein in it). When wear and tear is accelerated, 'dander' loss or "scaling" increases. The epidermis gets its strength from the protein keratin, which also makes up the hair and nails. Keratin-making cells do more than just make durable keratin however. They also produce chemicals called cytokines that are important for a healthy inflammatory response when the skin is injured.
Some of the surface or epithelial cells also produce a pigment called melanin, that gives colour to the skin and hair. Increases or decreases in melanin production occur with some diseases and a tumour of these cells causes a ‘melanoma’ an unpleasant tumour in both cats and dogs.
Throughout the skin are the cells of the immune system. These actively multiply when needed, causing inflammation, necessary for skin repair. Other immune system cells such as mast cells literally explode when irritated, releasing chemicals that provoke scratching, itching and rubbing. Sometimes skin thickens as a defensive response. Elbow calluses for example, common on heavy older dogs, reduce the likelihood of skin breaking and pathogens getting through.