A tattoo is the deposition of exogenous pigment into the skin or the accidental entry of pigmented material. Accidental tattooing may occur after abrasion injuries introducing asphalt, graphite, or carbon into the injured skin. Rare iatrogenic tattoos have developed after the use of ferrous subsulfate solution (Monsel solution) for coagulation purposes. While traumatic tattoos are not rare, decorative tattoos are more common.
Decorative tattooing has been practiced for thousands of years for artistic and cosmetic purposes. Tattoos are executed by professional tattoo artists using an electric needle to introduce particles of pigment into the dermis. The amateur tattoo artist may use any pointed object to place particles of ink. Tattooing remains a common custom in various countries and cultures. It gained popularity in many western countries throughout the 1990s. In the past, decorative tattoos were primarily seen in men, especially in groups such as sailors and members of the armed forces; however, they are increasingly observed in young professionals of both sexes. The components of many tattoo pigments have been identified, although new formulations continue to be formulated (see Tattoo Pigments).
Complications resulting from decorative tattoos are rare, but the incidence is increasing. [1, 2, 3, 4] The introduction of foreign substances into the skin can result in a range of adverse effects, including a toxic or immunologic reaction to the tattoo pigments, transmission of infectious disease, and the localization of skin disease within the tattoo. The immunologic reaction to tattoos can vary from an acute inflammatory reaction to allergic hypersensitivity. The histopathologic pattern can be granulomatous, lichenoid, or pseudolymphomatous.
See the image below.
Tattoo removal may be accomplished by a variety of methods and may be initiated whether the tattoo is responsible for an adverse reaction in the skin or for significant feelings of regret. Also see Tattoo Lasers.
Transmission of Infection
Although significant infection secondary to tattoos is currently unusual, infection may be introduced into the skin during the breach of the epidermal barrier. Pyogenic infection resulting in erysipelas and gangrene were a problem in the past, but localized skin infection secondary to gram-positive bacteria is seldom observed today. Transmission of tuberculosis, syphilis, leprosy, hepatitis, and HIV has also been recorded. By using a previously used and infected tattoo needle, inoculation and person-to-person transmission of viruses, including vaccinia and human papilloma virus, have been reported. [5, 6]
Certain types of tattoo inks and potential contamination of tattoo inks can cause reactions that range from relatively mild to very severe, particularly in individuals with preexisting comorbidities (cardiovascular disease, diabetes, immune compromise). On August 7, 2014, the US Food and Drug Administration issued a consumer warning about the potential health hazards associated with tattoos following reports of infections from a line of home tattoo kits. Testing revealed bacterial contamination in unopened tattoo ink bottles. 
See the image below.
Types of Reactions
Acute inflammatory reactions
Acute inflammatory reactions are associated with physical tissue injury and the injection of pigment dyes or metals into the skin. This reaction usually recedes without consequence within 2-3 weeks and is an expected adverse effect of the tattooing process.
Eczematous hypersensitivity reactions
Once acute inflammatory changes have resolved, the most frequent reaction observed with tattoos is an allergic sensitivity to one of its pigments. Individuals may manifest sensitivity to a particular pigment in several ways. Histopathologic evaluation of involved skin may reveal a spongiotic, granulomatous, or lichenoid type of tattoo reaction.
Commonly, hypersensitivity reactions to a tattoo pigment result in a contact dermatitis or photoallergic dermatitis. These conditions may manifest clinically as localized eczematous eruptions or, rarely, as an exfoliative dermatitis. Histopathologic findings include acanthosis, spongiosis, and a lymphocytic perivascular infiltrate.
See the image below.
Allergic reactions to red tattoo pigments are the most common and may be caused by a variety of pigments, especially mercury sulfide (cinnabar).  Patch testing may be positive for mercuric chloride but is not reliable for cinnabar. Alternative red dyes have been developed because of the problems associated with red tattoo pigment containing mercury; however, red tattoo reactions continue to be reported.
See the images below.
Organic vegetable dyes are used more often than red pigments containing mercury and have been associated with eczematous reactions and negative patch test results. Because organic dyes are insoluble, little penetration into the epidermis occurs; therefore, a negative patch test result occurs. Allergic reactions may occur when organic dyes are injected into the dermis during tattoo placement. One report describes allergic contact dermatitis from a temporary henna tattoo. 
See the image below.
Photo-aggravated reactions are most commonly caused by yellow (cadmium sulfide) tattoo pigment. Edema and erythema may develop upon exposure to sunlight. Although the mechanism is not clear, cadmium sulfide is the light-sensitive material used in photoelectric cells; therefore, the reaction is believed to be phototoxic. Red tattoos have been associated with photo-aggravated tattoo reactions less frequently than yellow tattoos, and most likely, these reactions are related to the trace amounts of cadmium added to brighten the red pigment.
In contrast to hypersensitivity reactions to red tattoos, reactions to pigments used to create green, blue, and black tattoos are much less common. Chromium in green tattoo pigment is associated with localized eczematous reactions at the site of the pigment, eczema of the hands, and generalized eczematous reactions. Patients may be sensitized primarily by exposure to cement. Patch testing to 0.5% potassium dichromate is often positive. Previously quiescent green-colored tattoos may become inflamed during patch testing in potassium dichromate–sensitive individuals. 
Blue tattoos that contain cobalt have been linked to localized hypersensitivity reactions and (rarely) spontaneous development of uveitis.
Granulomatous reactions may take 2 forms. Foreign body reaction to pigment can produce numerous pigment-filled giant cells. This reaction may be viewed as the counterpart to an irritant contact dermatitis. An immunologic granulomatous reaction is characterized by aggregates of epithelioid cells, a ring of lymphocytes, and a few giant cells. An immunologic granulomatous reaction may be indistinguishable from tattoo involvement seen as a Köbner response in sarcoidosis; therefore, further investigation, such as a complete review of systems and possibly chest radiography, may be warranted to exclude systemic disease.
Most commonly, mercury (red pigment) is associated with a granulomatous tattoo reaction; however, several reactions involving chromium (green pigment) and cobalt (blue pigment) have also been reported.  In contrast to an eczematous hypersensitivity tattoo reaction in which patch test results may be positive, granulomatous tattoo reactions are most likely associated with negative patch test results.
Manganese reportedly causes a granulomatous reaction in purple tattoos, but sufficient evidence is not available to establish manganese as the definitive etiologic agent. [17, 18] One patient was documented with a granuloma occurring in the violet areas of a tattoo shown to contain aluminum particles.  Intradermal provocation testing to aluminum was positive in this patient.
Granulomatous reactions have been reported in permanent eyeliner tattoos in several patients. Treatment is difficult because of the proximity to the lid margin. 
Lichenoid hypersensitivity tattoo reactions are less common than eczematous reactions. Evidence exists that the lichenoid reaction is an expression of delayed hypersensitivity to a lymphocytic T-cell infiltrate, which may simulate the graft-versus-host response. Mercury (red pigment) is responsible for most lichenoid tattoo reactions. Clinically, warty papules or plaques typical of hyperkeratotic lichen planus are usually confined to the red portion of the tattoo, although a generalized lichen planus reaction developed in one patient with a history of a lichenoid tattoo reaction.  This patient had occupational exposure to mercury.
Histopathologically, the pattern of inflammation is consistent with that of lichen planus, including a bandlike infiltrate at the dermoepidermal junction, liquefaction degeneration, hyaline bodies, and sawtooth rete ridges. Similar to granulomatous tattoo reactions, lichenoid reactions are associated with negative patch test results.
Delayed hypersensitivity to tattoo pigment may result in a pseudolymphomatous reaction. Lymphadenosis benigna cutis (pseudolymphoma) can develop after a variety of foreign substance exposures including insect bites, acupuncture, antigen injections, earrings, and tattoos.  Tattoo-induced pseudolymphoma occurs primarily within red portions of the tattoo. This reaction is rarely induced by green or blue tattoos.
Clinically, most reactions are characterized by flesh-to-plum or plum-red indurated nodules and plaques similar to cutaneous B-cell lymphoma. Be alert to this type of reaction to tattoo pigment to prevent an erroneous diagnosis of lymphoma. Pseudolymphoma may be distinguished from cutaneous lymphoma at histologic examination. Important features of a pseudolymphoma include germinal centers, a mixed cell infiltrate, prominent vasculature, and predominant involvement of the upper dermis compared with the lower dermis.
Immunohistochemical studies may provide additional information that helps distinguish pseudolymphoma from malignant B-cell proliferation in the skin. Monotypic light-chain expression highly suggests malignant lymphoma, while the lymphocytes in pseudolymphoma primarily are polyclonal.
Localization of Disease in Tattoos
Several cutaneous disorders show a predilection for tattooed skin. The eruption may represent the initial manifestation of the disease or the accentuation of an existing disorder in the area of the tattoo. Lichen planus, psoriasis, sarcoidosis, and lupus erythematosus have been associated with localization to the site of a tattoo.  No color predilection is demonstrated in this localization response. Whether disease localized to the tattoo represents the Köebner phenomenon or results from a locus minoris resistentiae that predisposes the area to disease is unclear.
Several patients with malignant melanoma occurring in a tattoo have been reported.  Most of these documented cases include melanomas found in tattoo sites used for marking an irradiation field. The combination of India ink and irradiation may be responsible for carcinogenesis. The association may be coincidental in the patients without predisposing injury.
Keratoacanthoma, a keratinizing squamous cell neoplasm of unknown origin characterized by rapid growth, have been observed with various types of skin injury. Although rare, eruptive keratoacanthoma have been reported in both red and multicolored tattoos. [32, 33]
Treatment & Management
Tattoo removal may be required when complications develop. If an allergic reaction is noted, some cases resolve with topical or intralesional steroids, but permanent removal may be necessary. 
Most often, tattoo removal is sought for cosmetic reasons. Decorative tattoo removal can be accomplished by a variety of methods. Treatment modalities used in the past included cryosurgery, dermabrasion, carbon dioxide lasers, and cold steel surgical excision. These destructive methods replaced the tattoo with a permanent scar. In 1964, Goldman et al were the first to describe the removal of tattoos with the Q-switched ruby laser.  These experiments were successful; however, the technique did not gain popularity until the 1990s. Laser tattoo removal often requires multiple treatments, and complete resolution of color may not be achieved in all cases.
Laser tattoo removal targets tattoo pigment-containing structures in the skin using the theory of selective photothermolysis. To achieve selective photothermolysis, sufficient energy to damage the target needs to be delivered with a pulse duration that is less than the thermal relaxation time of the target, which is itself defined by the size and shape of the target. The smaller the target, the shorter the thermal relaxation time and therefore the shorter the laser pulse duration required. The thermal relaxation time of tattoo pigment particles is less than 10 nanoseconds, suggesting that energy delivery in the picosecond range (1 trillionth of a second, 10-12 seconds) should be effective.
Historically, the Q-switched lasers (nanosecond duration, 10-9 seconds) ruby (694 nm), Q-switched Nd:YAG (1064 nm), Q-switched alexandrite (755 nm), Q-switched frequency-doubled Nd:YAG (532 nm), and the pigmented lesion pulsed dye (510 nm) lasers have been used to remove various tattoo pigments.
Recently developed picosecond laser systems generate significant photomechanical effects, which leads to a more effective mechanical breakup of the ink or pigment particles. Using picosecond pulses allows lower treatment fluences to be used, while keeping the peak tensile stress induced substantially higher than that typically produced with Q-switched lasers.
Selection of the appropriate wavelength is determined by the colors of tattoo pigment to be targeted. Based on “complementary matching” of tattoo pigment, green tattoo pigment is most effectively removed by 694-nm or 755-nm wavelengths and red tattoo pigment is best removed with 532-nm wavelength. Blue and black pigments are removed with any wavelength. More difficult to remove are yellow and orange pigments, but these are often treated with 532-nm wavelength.
Before laser treatment, tattoo pigment is localized within perivascular fibroblasts, mast cells, and macrophages. After treatment with a Q-switched or picosecond laser, rapid thermal expansion fragments the pigment-containing cells causing the pigment to become extracellular.
The release of ink particles into the extracellular space allows for lymphatic drainage and rephagocytosis of smaller residual ink particles. To a lesser extent, elimination of pigment by formation of scale-crust may be involved.
Complications following laser treatment of tattoos have been well documented. Laser treatment infrequently may cause a localized tattoo reaction to become generalized. [36, 37, 38] Pigment released into the extracellular space after laser treatment may be viewed as foreign by the immune system, causing a hypersensitivity response. 
Laser-induced photochemical changes can occur in tattoo inks resulting in irreversible immediate darkening of the tattoo. Pigments, including red, brown, and white, used for cosmetic tattoos are at highest risk for this type of reaction. The darkening reaction may be masked by an immediate whitening action after laser treatment. The immediate whitening of the tattoo is likely caused by the formation of gas bubbles that intensely scatter light. A small test area is advised because this transient whitening reaction may obscure the darkening reaction.  Most cosmetic tattoos contain iron (or titanium) oxide inks that, on laser irradiation, are reduced from ferric oxide to the ferrous oxide form; the latter being black and insoluble. Although this reaction pattern can usually be improved with continued laser treatments or vaporization with a carbon dioxide laser, the darkened color may be permanent.
The composition of tattoo pigment colors is as follows:
Black - Carbon (India ink), iron oxide, logwood
Blue - Cobalt aluminate
Brown - Ferric oxide
Green - Chromic oxide, lead chromate, phthalocyanine dyes
Purple - Manganese, aluminum
Red - Mercuric sulfide (cinnabar), sienna (ferric hydrate), sandalwood, brazilwood, organic pigments (aromatic azo compounds)
White - Titanium oxide, zinc oxide
Yellow - Cadmium sulfide