Characterized by dermal edema (wheal) and erthema (flare) also known as hives. Hive lesions typically last less than 24 and are usually pruritic. Hives can appear on anywhere on the body and they may change shape, move around, disappear and reappear over short periods of time.

Acute urticaria (short-term): can develop suddenly and will last "less" than 6 weeks. About 1 in 6 people will have acute hives at one point in their life.

Chronic urticaria (long-term): can develop suddenly and will persist "more" than 6 weeks. This type of urticaria is uncommon and occurs in only 0.1% of the population. 20% of people with chronic urticaria report still having problems 10 years after its onset.

This type of hives can occur right away, precisely after a pressure stimulus or as a deferred response to sustained pressure being enforced to the skin. In the deferred form, the hives only appear after about six hours from the initial application of pressure to the skin. Under normal circumstances, these hives are not the same as those witnessed with most urticariae. Instead, the protrusion in the affected areas is typically more spread out. The hives may last from eight hours to three days. The source of the pressure on the skin can happen from tight fitted clothing, belts, clothing with tough straps, walking, leaning against an object, standing, sitting on a hard surface, etc. The areas of the body most commonly affected are the hands, feet, trunk, abdomen, buttocks, legs and face. Although this appears to be very similar to dermatographism, the cardinal difference is that the swelled skin areas do not become visible quickly and tend to last much longer. This form of the skin disease is, however, rare.

Dermatographic urticaria (also known as dermatographism or "skin writing") is marked by the appearance of weals or welts on the skin as a result of scratching or firm stroking of the skin. Seen in 4–5% of the population, it is one of the most common types of urticaria, in which the skin becomes raised and inflamed when stroked, scratched, rubbed, and sometimes even slapped.

The skin reaction usually becomes evident soon after the scratching, and disappears within 30 minutes. Dermatographism is the most common form of a subset of chronic hives, acknowledged as "physical hives".

It stands in contrast to the linear reddening that does not itch seen in healthy people who are scratched. In most cases, the cause is unknown, although it may be preceded by a viral infection, antibiotic therapy, or emotional upset. Dermographism is diagnosed by taking a tongue blade and drawing it over the skin of the arm or back. The hives should develop within a few minutes. Unless the skin is highly sensitive and reacts continually, treatment is not needed. Taking antihistamines can reduce the response in cases that are annoying to the patient.

When the body is exposed to the cold in individuals afflicted by the condition, hives appear and the skin in the affected area typically becomes itchy. Hives result from dilation of capillaries which allow fluid to flow out into the surrounding tissue which is the epidermis.They resolve when the body absorbs this fluid. The border of a hive is described as polycyclic, or made up of many circles, and changes as fluid leaks out and then is absorbed. Pressing on a hive causes the skin to blanch distinguishing it from a bruise or papule. Hives can last for a few minutes or a few days, and vary from person to person. Also, a burning sensation occurs. During a severe reaction, low blood pressure, which can be life-threatening, can occur. A serious reaction is most likely to occur if the hives occur with less than 3 minutes of exposure (during a cold test).

The symptoms of aquagenic urticaria or water allergy (being allergic to water) are similar to all the other types of physical hives that are caused by various things. This may include wheals or small raised hives, intense itching, skin flushing in the areas that are prone to water contact. The symptoms may appear within minutes after the body comes in contact with the water. Aquagenic urticaria is a rare condition in which itchy urticaria (hives) develop rapidly after the skin comes in contact with water, regardless of its temperature. The hives associated with aquagenic urticaria are typically small (approximately 1–3 mm), red- or skin-colored welts (called wheals) with clearly defined edges. The rash most commonly develops on the neck, upper trunk and arms, although it can occur anywhere on the body. Some people have itching too. Once the water source is removed, the rash generally fades within 30 to 60 minutes.

Contact dermatitis is a localized rash or irritation of the skin caused by contact with a foreign substance. Only the superficial regions of the skin are affected in contact dermatitis. Inflammation of the affected tissue is present in the epidermis (the outermost layer of skin) and the outer dermis (the layer beneath the epidermis).

Contact dermatitis results in large, burning, and itchy rashes. These can take anywhere from several days to weeks to heal. This differentiates it from contact urticaria (hives), in which a rash appears within minutes of exposure and then fades away within minutes to hours. Even after days, contact dermatitis fades only if the skin no longer comes in contact with the allergen or irritant. Chronic contact dermatitis can develop when the removal of the offending agent no longer provides expected relief.

Irritant dermatitis is usually confined to the area where the trigger actually touched the skin, whereas allergic dermatitis may be more widespread on the skin. Symptoms of both forms include the following:

- Red rash. This is the usual reaction. The rash appears immediately in irritant contact dermatitis; in allergic contact dermatitis, the rash sometimes does not appear until 24–72 hours after exposure to the allergen.

- Blisters or wheals. Blisters, wheals (welts), and urticaria (hives) often form in a pattern where skin was directly exposed to the allergen or irritant.

- Itchy, burning skin. Irritant contact dermatitis tends to be more painful than itchy, while allergic contact dermatitis often itches.

While either form of contact dermatitis can affect any part of the body, irritant contact dermatitis often affects the hands, which have been exposed by resting in or dipping into a container (sink, pail, tub, swimming pools with high chlorine) containing the irritant.

Secondary cold contact urticaria is a cutaneous condition characterized by s, due to serum abnormalities such as cryoglobulinemia or cryofibrinogenemia are extremely rare, and are then associated with other manifestations such as Raynaud's phenomenon or purpura.

Dermatographic urticaria manifests as an allergic-like reaction, in general a warm red wheal (welt) to appear on the skin. It can often be confused with an allergic reaction to the object causing the scratch, when in fact it is the act of being scratched that causes a wheal to appear. These wheals are a subset of [urticaria] (hives) that appear within minutes, in some cases accompanied by itching. The first outbreak of urticaria can lead to others on body parts not directly stimulated, scraped, or scratched. In a normal case, the swelling will decrease with no treatment within 15–30 minutes, but, in extreme cases, itchy red welts may last anywhere from a few hours to days.

The exact underlying cause of aquagenic urticaria is poorly understood.

Symptoms are thought to be the result of histamine being released by mast cells on the surface of the skin. Due to the lack of antigens, histamine causes the skin to swell in affected areas. If the membrane that surrounds the mast cells is too weak it will easily and rapidly break down under physical pressure, which will therefore cause an allergic-like reaction.

Symptoms can be caused or induced by

- stress

- tight or abrasive clothing

- watches

- glasses

- heat

- cold

- anything placing pressure on exposed skin

- infection

The underlying cause of dermatographism is not known, and can last for many years without relief. The condition may subside and be effectively cured; however, it is often a lifelong ailment. It is not a life-threatening disease and is not contagious.

Dermographism may occur in Mastocytosis (systemic mast cell proliferation).

Generally, the areas affected are exposed skin not usually protected by clothing; however it can also occur in areas covered by clothing. Areas constantly subjected to the sun's rays may only be slightly affected if at all. People with extreme cases will also have reactions to light bulbs that emit a UV wavelength (any bulb that is not an LED). Parts of the body only thinly covered can also potentially be subjected to an outbreak.

Life with SU can be difficult. Patients are subject to constant itching and pain, as within minutes of the initial exposure to UV radiation a rash will appear. The urticarial reaction begins in the form of pruritus, later progressing to erythema and edema in the exposed areas of the skin. If vast areas of the body are affected, the loss of fluid into the skin could lead to light-headedness, headache, nausea, and vomiting. Extremely rarely, patients have been reported to experience an increase in heart rate that can cause a stroke or heart attack due to the body cavity swelling. Other rare side effects can be bronchospasm and glucose instability issues. Also, if a large area of the body is suddenly exposed the person may be subject to an anaphylactic reaction. Once free of exposure, the rash will usually fade away within several hours; rare and extreme cases can take a day or two to normalize depending on severity of the reaction.

The hives are observed to coincide with perspiration points of sweating.

Solar urticaria (SU) is a rare condition in which exposure to ultraviolet or UV radiation, or sometimes even visible light, induces a case of urticaria or hives that can appear in both covered and uncovered areas of the skin. It is classified as a type of physical urticaria. The classification of disease types is somewhat controversial. One classification system distinguished various types of SU based on the wavelength of the radiation that causes the breakout; another classification system is based on the type of allergen that initiates a breakout.

The agent in the human body responsible for the reaction to radiation, known as the photoallergen, has not yet been identified. The disease itself can be difficult to diagnose properly because it is so similar to other dermatological disorders, such as polymorphic light eruption or PMLE. The most helpful test is a diagnostic phototest, a specialized test which confirms the presence of an abnormal sunburn reaction. Once recognized, treatment of the disease commonly involves the administration of antihistamines, and desensitization treatments such as phototherapy. In more extreme cases, the use of immunosuppressive drugs and even plasmapheresis may be considered.

The initial discovery of the disease is credited to P. Merklen in 1904, but it did not have a name until the suggestion of "solar urticaria" was given by Duke in 1923. However, their research contributed to the study of this uncommon disease. More than one hundred cases have been reported in the past century.

This subtype of CU refers to those who have abnormally reduced sweating.

Contact dermatitis is a type of inflammation of the skin.

It results from either exposure to allergens (allergic contact dermatitis) or irritants (irritant contact dermatitis). Phototoxic dermatitis occurs when the allergen or irritant is activated by sunlight. Diagnosis of allergic contact dermatitis can often be supported by patch testing.

A sweat allergy is the exacerbation of atopic dermatitis associated with an elevated body temperature and resulting increases in the production of sweat. It appears as small reddish wheals that become visible in response to increased temperature and resulting production of sweat. It can effect all ages. Sweating can trigger intense itching or cholinergic urticaria. The protein MGL_1304 secreted by mycobiota present on the skin such as "Malassezia globosa" acts as a histamine or antigen. People can be desensitized using using their own samples of sweat that have been purified that contains small amounts of the allergen. The allergy is not due to the sweat itself but instead to an allergy-producing protein secreted by baceria found on the skin.

Cholinergic urticaria (CU) is one of the physical urticaria which is provoked during sweating events such as exercise, bathing, staying in a heated environment, or emotional stress. The hives produced are typically smaller than classic hives and are generally shorter-lasting.

Multiple subtypes have been elucidated, each of which require distinct treatment.

Tannic-acid has been found to suppress the allergic response along with showering.

Urticaria pigmentosa is characterized by excessive amounts of mast cells in the skin. Red or brown spots are often seen on the skin, typically around the chest and forehead. These mast cells, when irritated (e.g. by rubbing the skin, heat exposure), produce too much histamine, triggering an allergic reaction that leads to hives localized to the area of irritation, sometimes referred to as Darier's sign. Severe itching usually follows, and scratching the area only serves to further symptoms. Symptoms can be mild (flushing and hives that require no treatment), moderate (diarrhea, tachycardia, nausea/vomiting, headache, and fainting), or life-threatening (vascular collapse requiring emergency treatment and hospitalization).

Urticarial dermatoses are distinct from urticaria, which examples being drug-induced urticaria, eosinophilic cellulitis and bullous pemphigoid. It is important to distinguish urticaria from urticarial dermatoses. The individual wheals of urticaria are ‘here today and gone tomorrow’ (i.e. they last less than 24 hours), whereas with urticarial dermatoses, the individual lesions last for days or longer.

Urticaria pigmentosa (also known as generalized eruption of cutaneous mastocytosis (childhood type)

) is the most common form of cutaneous mastocytosis. It is a rare disease caused by excessive numbers of mast cells in the skin that produce hives or lesions on the skin when irritated.

Clinical examination alone cannot distinguish between a response caused by infection, such as cellulitis, and skeeter syndrome. However, skeeter syndrome usually progresses over the course of hours versus cellulitis, which typically will evolve over the course of several days. As such, accurate history is imperative when making the diagnosis. Since IgE and IgG are key players in mosquito allergy, diagnosis can be confirmed by an immunosorbent assay measuring IgE and IgG to mosquito saliva antigens.

Although there are a multitude of varying appearances, the id reaction often presents with symmetrical red patches of eczema with papules and vesicles, particularly on the outer sides of the arms, face and trunk which occur suddenly and are intensely itchy occur a few days to a week after the initial allergic or irritant dermatitis. Most commonly, athletes foot can lead to localised vesicles on hands, bacterial infections to erythema nodosum and herpes simplex virus to erythema multiforme.

The diagnosis is frequently made by treating the initial triggering skin problem and observing the improvement in the eczematous rash. Both the initial skin problem and the id reaction must be observed to make the diagnosis.

All dyshidrotic rashes are not id reactions, but id reactions are often dishydrotic-like.

Initial tests may include isolating a fungus by taking a swab and sending it for culture. Patch testing may be considered if there is suspicion of allergic contact dermatitis.

A skin biopsy is rarely necessary, but if done mostly shows an interstitial granulomatous dermatitis, some lesions being spongiotic. Id reactions cannot be distinguished from other skin diseases by histopathology. However, they can be distinguished from other id reactions by histopathology.

Type I hypersensitivity (or immediate hypersensitivity) is an allergic reaction provoked by reexposure to a specific type of antigen referred to as an allergen. Type I is not to be confused with type II, type III, or type IV hypersensitivities, nor is it to be confused with Type I Diabetes or Type I of any other disease or reaction.

Exposure may be by ingestion, inhalation, injection, or direct contact.

Causes include infection with dermatophytosis, Mycobacterium, viruses, bacteria and parasites. Eczematous conditions including contact allergic dermatitis and stasis dermatitis as well as stitches and trauma have also been associated with id reactions. Radiation treatment of tinea capitis has been reported as triggering an id reaction.

Treatment usually involves adrenaline (epinephrine), antihistamines, and corticosteroids.

If the entire body is involved, then anaphylaxis can take place, which is an acute, systemic reaction that can prove fatal.