Type IV (cellular immune–mediated or delayed-hypersensitivity) reactions involve recognition of antigen by sensitized T cells. Antigen-presenting cells form peptides that are expressed on the cell surface in association with major histocompatibility complex class II molecules. Memory T cells recognize the antigen peptide/major histocompatibility complex class II complexes. Cytokines, such as interferon-γ, tumor necrosis factor-α, and granulocyte-macrophage colony-stimulating factor, are secreted from this interaction, which activates and attracts tissue macrophages. Contact allergies (nickel, poison ivy, topical medications) and immunity to tuberculosis are type IV reactions.
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A family history of allergic disease is often present in affected patients. Multiple genes predispose to atopy. If one parent has allergies, the risk that a child will develop an allergic disease is 25%. If both parents have allergies, the risk increases to 50% to 70%. Similar atopic diseases tend to occur in families.
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Children with allergic rhinitis exhibit frequent nasal itching and rubbing of the nose with the palm of the hand, the allergic salute, which can lead to a transverse nasal crease found across the lower bridge of the nose. Allergic shiners, blue-gray to purple discoloration below the lower eyelids that is attributed to venous congestion, along with swollen eyelids or conjunctival injection are often present in children. Dermatologic findings of atopy include hyperlinearity of the palms and soles, white dermatographism, pityriasis alba, prominent creases under the lower eyelids (Dennie−Morgan folds or Dennie lines), and keratosis pilaris (asymptomatic horny follicular papules on the extensor surfaces of the arms).
Cutaneous manifestations are most common and range from generalized xerosis (dry skin) to urticaria to the pruritic, erythematous papules, and vesicles of atopic dermatitis. There may be involvement of the upper respiratory tract with allergic rhinitis and the lower respiratory tract with asthma. Allergic disease may involve only the skin or the nose, eyes, lungs, and gastrointestinal tract alone or in combination. It is distinguished by environmental exposure to an inciting trigger and usually a history of previous similar disease or development of symptoms after a suspected trigger. Many patients have more than one allergic symptom.
Atopy is characterized by elevated levels of IgE (Table 77-2)
Available @ StudentConsult.com and eosinophilia (3% to 10% of white blood cells or an absolute eosinophil count of >250 eosinophils/mm3) with a predominance of Th2 cytokines, including interleukin (IL)-4, IL-5, and IL-13. Extreme eosinophilia suggests a nonallergic disorder such as infections with tissue-invasive parasites, drug reactions, or malignancies (Table 77-3). A classic example of a type IV reaction is the tuberculin skin test. A small amount of purified protein derivative from Mycobacterium tuberculosis is injected intradermally (see Chapter 124). In a previously sensitized individual, a type IV inflammatory reaction (induration) develops over the next 24 to 72 hours.