When we refer to "the skin" it may seem as if we mean something like the wrapping around a hot dog—a covering that keeps the body’s contents in place. Human skin is a lot more than that. In fact, the skin is an organ with multiple functions, just like other complex organs such as the liver, lungs, kidneys and gastrointestinal tract. Also similar to other organs, there are many ways in which one or more of the multiple functions of skin can go wrong and cause disease.

Unlike a hot dog skin, human skin has a complex, multi-layered structure. The complex, intricate structure allows the skin to have many complex functions. Multiple functions would not be possible without complex structure.

Each layer of the skin expresses its own unique reaction when disturbed. Almost all the reactions are the result of inflammation occurring in that layer. Since each layer is made up of different types of cells, the inflammation that occurs in those layers appears differently.

The three basic layers of skin are:

Epidermis—outermost layer

Dermis—middle layer

Subcutaneous tissue—deep layer

The epidermis is made up almost entirely of keratinocytes, tough-walled cells loaded with many chemicals waiting to be released when they are "turned on" by various insults—e.g., sunlight, bacteria, fungi, viruses, irritants. Each insulting "trigger" causes the release of appropriate chemicals which signal to each underlying skin layer which type of inflammation is needed for protection against that "insult."

Dispersed among the keratinocytes are Langerhans cells, whose job it is to deliver antigens (for example, allergens or other foreign material like germs) that come in contact with the skin to the immune system. These Langerhans cells go to the draining lymph nodes where the inflammatory cells are marshaled for action, if the immune system recognizes the antigen as potentially harmful.

Scattered in the lowest, or basal, layer of the epidermis are melanocytes that produce the brown pigment responsible for skin color.

The most common type of inflammation in the epidermis is usually recognized as "eczema." Since "eczema" is the result of inflammation, all eczemas look alike under the microscope whether they are "bubbly rashes" resulting from contact with poison ivy, other contact dermatitis, atopic dermatitis, etc.

The dermis is the major part of the skin, upon which the epidermis is attached. It is made up of collagen fibers, the tough protein connective tissue of skin, joints and bone, cells and other molecules. Within the collagen network, we find sweat glands, nerve fibers and blood vessels. The only cells naturally found in the dermis are fibroblasts (which produce the collagen), and mast cells, those most essential cells which are the body’s "first line of defense" against any insult to the skin. Mast cells are the fast-acting decision-makers, controlling the redness, the itch, and swelling of inflammation. They also summon the help of other inflammatory cells (basophils, eosinophils, and T-cell lymphocytes) which can be helpful in suppressing the insult but may actually augment the inflammation. The lymphocytes are released into the circulation from the lymph glands and enter the dermis to "home" to the site where the skin was perturbed. They are essential for the appearance of eczema.

This deepest layer of the skin is where we store most of our body fat. It serves to keep us warm, and allows the upper layers of skin to move freely with body movements. Running through the fat are the nerves and blood vessels that serve the skin.

The more we learn about the skin, the more spectacular we find it to be. Until recently, the function of skin was believed to be only that of a "barrier" that kept the outside world from getting in, and preventing the contents of our body from leaking out. It was not until the last 25 years that we appreciated all the other things our skin does. It is now recognized as our body’s largest immunologic organ. It is the immunologic mechanisms that are essential in protecting us from sunlight, bacteria, fungi, viruses and all the things that do not belong in our bodies.

When this complex immune environment functions optimally, the body is protected from environmental attacks. When something happens to cause an immune system imbalance or dysregulation, processes may be initiated that become pathologic (disease causing). Among these processes are those that cause eczematous reactions.

Many things can cause the dysregulation of our immune mechanisms. The most common test of our immune mechanisms occurs whenever we come in contact with things the body identifies as not belonging on the skin. Perhaps the most common example is "chapping" of the hands caused by having the hands too much in water. Too much water in the epidermis damages the epidermis, resulting in the release of "pro-inflammatory" chemicals; when more severe, the reaction may present as (hand) "eczema." "Poison ivy" causes another type of inflammation, an allergic reaction, that often triggers an "over-reaction" resulting in the itchy, bubbly (eczematous) rash. The "itch" of poison ivy results in a desire to scratch, and scratching is a primitive and often undesirable way to remove the allergen from the skin.

A more complicated, but common, dysregulation is noted in people who are born with an exaggerated reaction to common things in the environment. They are readily identified by a family history of being "allergic." Medically, they are called "atopic," a word borrowed from the Greek and meaning atypical or strange. Since we now know that there are more than 20 genes involved in this atopic state, we understand why no two patients have the same symptoms. Not only does the degree of involvement vary, but the organs affected change and vary. These patients can react "strangely" in the nose to produce hay fever, when the lungs are involved to produce asthma, and when the skin is involved to present with "hives" or atopic dermatitis. A patient can have one, two or all three of these problems.

Atopic patients "over-react" to things in their environment that cause no reaction in non-atopic individuals. For instance, when a non-atopic person inhales "dust," he/she may sneeze once or twice but the atopic (sensitized) person will sneeze many times and instead of "clearing the throat" may have a bronchospasm with wheezing and can become an asthma attack. Things that can cause these "over-reactions" include "allergens" such as pollen, molds and some foods, but often the "triggers" are not allergens but cold, heat, stress, irritants, etc. While the result of the various triggers seems to be the same, the inflammatory mechanisms are slightly different. Therefore, some patients could "get shots" to be hyposensitized but "shots" do nothing for the non-allergen induced reactions. Those triggers must be avoided in the immediate environment.

From the above, we can now understand much of the how and why of these problems in the atopic patient. Since the atypical reactions are inherited, it is understandable why these conditions "run in families," and why "allergies" in parents indicate the possibility for inheriting the genes involved in atopy. While we can more easily identify "allergens" that can trigger the over-reactions, we must relate what is common between the allergen-induced reactions and the reactions not induced by allergens. The most common denominator seems to be the atopic person having cells that can be "triggered" (activated) by either allergens or non-allergens—a condition not found in the non-atopic person. That cell, we believe, is the T-helper cell type 2 (Th2) cell. This white blood cell, when activated—either by allergen on non-allergen—releases chemicals that lead ultimately to inflammation in susceptible organs such as nose, lungs, or skin, causing itch, sneeze and wheeze.

The non-atopic individual has a predominance of different T cells identified as T-helper cells type 1 (Th1). When these cells are "triggered," a different variety of inflammatory chemicals are released. Allergic contact dermatitis, like "poison ivy," is the typical reaction caused in non-atopic people. T cells are noted in many diseases, and we are now learning that each of those diseases is caused by differences in the number of each type of T cells present (the Th1/Th2 ratio). In most people, there is a balance of Th1 and Th2 cells. Because Th1 cells inhibit Th2 cells and Th2 cells inhibit Th1 cells, most people do not develop an exaggerated, one-sided immune response that leads to an allergic reaction. That is, that the normal balance of these two cell types leads to a balanced immune response. An inability to achieve this balanced immune response can lead to skin diseases such as eczema.

"Eczema" is best defined as any "bubbly" rash and not as a specific disease. Diseases that may show eczema include: (1) contact dermatitis, allergic and irritant, (2) atopic dermatitis, (3) nummular eczema, (4) dyshidrotic eczema, and (4) many others.