A cat scratch often results in an immediate, raised, red bump on the skin, medically known as a wheal or a welt. This localized swelling develops quickly and is typically accompanied by temporary itching and redness. The formation of this raised area is a rapid biological defense mechanism. The body reacts to foreign substances deposited beneath the skin’s surface, triggering an immune response.
The Role of Cat Saliva and Dander
The primary cause of the immediate welt is the introduction of specific feline proteins into the minor break in the skin barrier. Cats constantly groom themselves, coating their fur, dander, and claws with saliva. The claws act as microscopic delivery systems for these foreign substances.
The most potent substance is the protein Fel d 1, produced in the cat’s sebaceous glands and saliva. Fel d 4, another significant allergen, is also present in the saliva. When the cat scratches a person, these minute protein particles are injected directly into the upper layers of the skin alongside environmental debris and dander. These proteins act as antigens, substances capable of eliciting an immune response.
The Immune System’s Histamine Reaction
The immune system, specifically in individuals sensitive to these feline proteins, rapidly mounts a defense against the perceived threat. Specialized immune cells called mast cells are stationed throughout connective tissues, including the dermis layer of the skin. Upon detecting the foreign Fel d 1 and Fel d 4 antigens, these mast cells are instantly activated.
This activation causes the mast cells to degranulate, meaning they release stored chemical mediators, the most relevant of which is histamine. Histamine initiates the localized inflammatory response by acting on the surrounding blood vessels, causing two immediate effects that lead to the characteristic welt.
Vasodilation and Permeability
First, histamine causes vasodilation, which is the widening of the small blood vessels near the scratch site. This influx of blood creates the redness and warmth observed around the wound. Second, histamine increases the permeability of the capillaries, causing the cells lining the blood vessels to separate slightly.
This separation allows fluid, plasma proteins, and other immune components to leak out of the bloodstream and into the surrounding tissue. This leakage of fluid into the skin tissue elevates the affected area, forming the distinct, raised wheal or welt. The histamine also stimulates nerve endings, which results in the intense itching sensation. This entire process is a form of immediate hypersensitivity, peaking quickly and typically resolving within a few minutes to a few hours as the histamine is metabolized and the fluid is reabsorbed.
Differentiating Welts from Bacterial Infection
Understanding the temporary, histamine-driven welt is important for distinguishing it from a more serious, delayed infectious process. The initial welt appears almost instantly and begins to fade within the same day, marking a non-threatening, localized allergic reaction. A true bacterial infection, however, does not manifest immediately; its symptoms appear hours or days after the scratch.
Signs that indicate a bacterial infection include increasing redness that spreads beyond the scratch line, throbbing pain, and warmth at the site that persists beyond 24 hours. The presence of pus or drainage from the wound also signals a developing infection. A deeper, rapidly progressing infection caused by bacteria like Pasteurella multocida can cause significant pain and swelling within 8 to 24 hours.
A specific concern is Cat Scratch Disease (CSD), caused by the bacterium Bartonella henselae. This infection has a distinct, delayed presentation, with initial symptoms appearing three to 14 days after the injury. The first sign is often a blister or raised lesion at the scratch site that does not heal, followed by the painful swelling of lymph nodes (lymphadenopathy) nearest the site of the scratch. This lymph node swelling, along with fever, fatigue, or headache, signals a systemic infection requiring medical consultation.