Allergic Contact Dermatitis to Tattoo Ink: This condition is a common reaction to tattoo ink, especially red ink, which can cause a delayed hypersensitivity reaction leading to raised, scaling, and lumpy skin along the lines of the tattoo. The reaction can occur years after the tattoo was applied.

Sarcoidosis: A systemic disease that can cause skin lesions, including those that are raised and lumpy, and can also cause skin discoloration. It might be considered if the lesions are not strictly limited to the tattoo area and if other systemic symptoms are present.
Granuloma Formation: A type of skin reaction that can occur in response to foreign substances, including tattoo ink. It presents as lumps under the skin and can be associated with scaling and redness.
Psoriasis: An autoimmune condition that can cause scaling skin and red plaques. While it's less directly related to tattoos, it could be considered if the patient has a history of psoriasis or if the lesions are not strictly confined to the tattooed areas.

Malignant Transformation (e.g., Squamous Cell Carcinoma or Melanoma): Although rare, it's crucial to consider the possibility of malignant transformation within a tattoo, especially if there are changing lesions or if the patient has a history of skin cancer.
Infectious Complications (e.g., Abscesses, Cellulitis): Infections can cause redness, warmth, swelling, and lumps. While the description suggests a more chronic process, acute infections need to be ruled out, especially if there's a history of recent trauma to the tattoo or signs of systemic infection.
Vasculitis: Inflammation of the blood vessels can cause a variety of skin manifestations, including lumps, bruising, and discoloration. It's a serious condition that requires prompt diagnosis and treatment.

Keratoacanthoma: A rare skin tumor that can appear as a lump and may resolve on its own. It's less likely but could be considered in the differential for lumps appearing on the skin.
Foreign Body Granuloma to Tattoo Pigment: Similar to granuloma formation but specifically refers to a reaction against the tattoo pigment itself, leading to lumps and inflammation.
Lichen Planus: An autoimmune condition that can affect the skin and mucous membranes, causing lesions that are often purple and itchy. It's less commonly associated directly with tattoos but could be considered in a broader differential for skin lesions.