Allergen immunotherapy involves subcutaneous injections of gradually increasing quantities of specific allergens to an allergic patient until a dose is reached that will raise the patient’s tolerance to the allergen over time, thereby minimizing symptomatic expression of the disease. Because the proteins and glycoproteins used in allergen immunotherapy are extracted from materials such as pollens, molds, pelt, and insect venoms, they were originally called allergen extracts. In 1998, the World Health Organization (WHO) proposed the term “allergen vaccine” to replace “allergen extract,” because allergen immunotherapy is an immune modifier just as vaccines are.¹

The efficacy of allergen immunotherapy has been known since 1911, when Noon injected an extract of grass pollen into a person in England whose allergic symptoms coincided with the pollination of grass.² Since then, controlled studies have shown that allergen immunotherapy is effective in patients with allergic rhinitis, allergic conjunctivitis, allergic asthma, and allergic reactions to Hymenoptera venom.^3–6 Patients with one or more of these diagnoses are considered for immunotherapy if they have well-defined, clinically relevant allergic triggers that markedly affect their quality of life or daily function, and if they do not attain adequate symptom relief with avoidance measures and pharmacotherapy. Despite proven efficacy, the exact mechanism of allergen immunotherapy remains unknown.

Selection of Patients

To make a definitive diagnosis of allergy, IgE-mediated, type I, immediate-hypersensitivity skin testing typically is performed by scratching diluted allergen into the skin surface or by injecting it intradermally. A positive skin test reaction reflects the presence of specific IgE antibodies to the tested allergen, and a correlation of the specific IgE antibodies with the patient’s symptoms, suspected triggers, and allergen exposure is definitive. In vitro, allergen-specific immunoassays to detect serum IgE antibodies are less sensitive than skin testing but may be used in patients with skin diseases that would obscure skin testing results or in those who cannot stop taking medications that suppress the skin test response. The circumstances in which allergen immunotherapy is particularly useful are summarized in Table 1. The allergens for which immunotherapy is known to be effective are Hymenoptera venom,⁵ pollens,^5,6 cat dander,⁷ dust mites,⁸ cockroaches,⁹ and fungi.¹⁰ Allergy immunotherapy is not efficacious for atopic dermatitis, urticaria, or headaches, and cannot be used for food allergies because the risk of anaphylaxis is too great.

Benefits

Durham and colleagues¹¹ conducted a randomized, double-blind, placebo-controlled trial to look at effects in patients who had received three to four years of immunotherapy. They were able to demonstrate a marked reduction in allergy symptom scores and antiallergic medication usage, as well as an alteration in the natural course of allergic disease. Preliminary reports suggest that immunotherapy for allergic rhinitis may reduce the risk for later development of asthma in children.^12,13 In addition, early treatment with allergen immunotherapy in children who were sensitive only to house dust mites reduced development of sensitivity to other allergens.¹⁴ In contrast to the use of antiallergic medication, allergen immunotherapy has the potential to alter the natural course of allergic disease and prevent progression or development of multiple allergies. Consequently, many allergists have suggested its use earlier in the course of allergic disease.

In 2000, the Immunotherapy Committee of the American Academy of Allergy, Asthma, and Immunology (AAAAI) provided a five-year cost comparison of medication usage and single-injection allergen immunotherapy for allergic rhinitis. The cost of medications is much greater than that of single-injection immunotherapy. Long-term costs deriving from the morbidity and complications of allergic diseases are not established, but allergies usually begin early in life and persist if not treated with allergen immunotherapy. A reasonable assumption is that allergen immunotherapy dramatically lowers the cost of treating allergic diseases.

Standardization, Storage, and Mixing of Allergen Vaccines

Ideally, vaccines should be standardized with a defined potency and labeled with a common unit.¹⁵ Such standardization would eliminate the variability in vaccines and allow for safer and more effective dosing. The Bioequivalent Allergy Unit (BAU), which is assigned by the U.S. Food and Drug Administration based on quantitative skin testing performed on a reference population of allergic patients known to be highly skin-test–reactive to that allergen, reflects clinical potency and is currently used for standardization of vaccines.

Standardized allergens available in the United States include cat dander, grass pollens, dust mites, and short ragweed pollen. Unstandardized vaccines may vary widely in biologic activity based on manufacturer and by lot, depending on the allergen content of the raw material and the conditions of extraction. Furthermore, the labeling conventions of Protein Nitrogen Units (PNUs) or weight by volume (wt/V) reflect protein content but not allergenic potency. Research is underway on new technologies for DNA and protein analysis that would allow an allergen vaccine to be characterized by the content of the major allergen and the consistency of each lot to be monitored accurately.

Vaccine strength is maintained by a number of procedures, including lyophilization and reconstitution with a stabilizer that contains an antimicrobial agent. A volume effect can occur as a result of adherence of the allergen to the vial surface; the larger the surface area of the vial, the more allergen is lost. Glycerol and human serum albumin (0.03 percent) are used to mitigate the volume effect. Glycerol has the added advantage of being an antimicrobial agent. At a concentration of 50 percent, glycerol inhibits enzymatic degradation of the allergens, but it may be irritating at this concentration. The combination of human serum albumin as a stabilizer and phenol as an antimicrobial additive often is used. However, human serum albumin typically is refused by patients who are Jehovah’s Witnesses.

Vaccines must be stored properly to preserve biologic activity (Table 2).^16,17 Vaccines should be refrigerated at 4°C (39.2°F) because storage at ambient room temperature results in loss of potency within weeks, with degradation occurring within days at higher temperatures. Critical to vaccine potency is the dilution effect: highly concentrated vaccines are more stable than dilute vaccines.¹⁸ The vaccine label should always be checked for the expiration date.

For immunotherapy to be effective, an optimal dose of each allergen must be determined. When a patient has multiple sensitivities caused by related and unrelated allergens, vaccines containing mixtures of these allergens may be prescribed. As multiple vaccines are mixed, not only will the concentration of each allergen be decreased, but certain allergens will interact. For example, fungi, dust mites, insect venoms, and cockroach have high proteolytic enzyme activity and may be combined with each other but should not be mixed with other allergens. Insect venoms usually are given alone.

Vaccine Administration

The maintenance concentrate is the dose of vaccine that is considered to be therapeutically effective for each of its constituent components. The maintenance concentrate should be determined by a prescribing allergist and clearly written on a standardized Maintenance Concentrate Prescription Form (available online athttp://www.aaaai.org).An optimal maintenance dose in the range of 5 to 20 mcg of major allergen per injection correlates with efficacy. Maintenance concentration is usually achieved by administering between 18 and 27 serial dose increments at weekly intervals (a build-up schedule written by the allergist) until the maintenance concentrate is achieved. In a typical build-up schedule, the patient will reach the maintenance concentrate in six months, but patients with a higher degree of allergen sensitivity may require a longer build-up phase. The maintenance dose usually is administered every three to four weeks, and maximum benefit typically is achieved in four to five years. Some patients will note early improvement in their symptoms, but long-term benefit seems to be related to the cumulative dose of vaccine given over time.

To reduce administration errors, the AAAAI recommends a universal, consistent, and redundant labeling system for every vial (Table 3).¹⁸

Some circumstances warrant adjustments in the dosage schedule. In these situations, communication between the family physician and the prescribing allergist is encouraged to increase safety and avoid unexpected reactions. If the interval between injections is prolonged (Table 4), the dose of vaccine must be reduced; when a new maintenance vial is obtained from the manufacturer, a dose reduction of 50 percent is recommended. For example, 0.5 mL of 1:500 V/V dilution should be reduced to 0.25 mL of 1:500 V/V dilution. The dose is increased every seven to 14 days until the maintenance dose is reached again. No evidence-based guidelines for dose adjustments following local, systemic, or delayed reactions are available, and the allergist should provide treatment suggestions for each of these reactions (Table 5).

Listed in Table 6 are items that should be reviewed before injecting the patient. The desired injection site is the outer aspect of the upper arm, midway between the shoulder and the elbow in the groove between the deltoid and triceps muscles. The injection is given subcutaneously, preferably with a 26- or 27-gauge needle; if blood is aspirated initially, the vaccine should not be injected. The plunger on the syringe should be depressed at a rate that does not result in wheal formation or excessive pain. Mild pressure should be applied to the injection site for about one minute, and a bandage may be placed if needed. Rubbing the injected area causes rapid absorption and should be avoided.

Safety Issues

Allergen immunotherapy is safe, but the potential for an adverse reaction is always present. Although these reactions are rare, they can be life-threatening. In 1924, Lamson reported the first case of death following immunotherapy.¹⁹ A statistical review of the literature about systemic reactions following allergen immunotherapy by Lockey and colleagues²⁰ found that severe systemic reactions occurred in less than 1 percent of the patients receiving conventional immunotherapy in the United States. From 1985 to 1993 in the United States, 52.3 million administrations of immunotherapy resulted in 35 deaths. These numbers equate to a mortality incidence of less than one per 1 million.²¹

Patients with medical conditions that reduce their ability to survive systemic allergic reactions are not candidates for allergen immunotherapy. Examples of such conditions include chronic lung disease with a forced expiratory volume in one second (FEV₁) of less than 50 percent, beta-blocker or angiotensin-converting enzyme (ACE) inhibitor therapy, unstable angina or myocardial infarction, uncontrolled hypertension, and major organ failure. Allergen immunotherapy also cannot be used in patients who would have difficulty reporting signs and symptoms of a systemic reaction, such as children younger than three or four years. In addition, beta-blocker or ACE-inhibitor therapy may mask early signs of anaphylaxis. Patients who have not been compliant with other forms of therapy are not likely to be compliant with immunotherapy, thus necessitating frequent alteration in dosage schedules and increasing the chance for errors. Patients should be assessed with each injection for newly acquired risks that may not have been present at the beginning of allergen immunotherapy.

Patients with severe, poorly controlled asthma are at higher risk for systemic reactions to immunotherapy injections than patients with stable, well-controlled asthma.²⁰ Some physicians measure peak expiratory flow readings in all patients with asthma before administering allergen immunotherapy and withhold injections if the reading is less than 70 percent of predicted. Other measures that should be performed to minimize the risk of adverse reactions to allergen immunotherapy are listed in Table 6.

Because a systemic reaction occurring during pregnancy may produce severe fetal hypoxia or precipitate premature uterine contractions, immunotherapy should not be initiated during pregnancy.¹⁸ However, immunotherapy can be maintained during pregnancy provided the patient is tolerating and benefiting from the injections. The immunotherapy dose should not be increased in a pregnant patient until after delivery.

Anaphylaxis is the most serious risk related to allergen immunotherapy. The vaccines must be administered in a setting with trained professionals who are equipped to recognize and treat anaphylaxis²² (Table 7). A retrospective study found that most systemic reactions occurred within 30 minutes of injection.²³ Hence, the current recommendation is to allow at least 20 to 30 minutes of observation following an injection. Patients who have had a systemic reaction after more than 30 minutes following an injection require longer observation; in addition, they should be given injectable epinephrine to carry and instructions about how to use it. Nonetheless, reactions may occur without warning signs or symptoms, and documentation of informed consent must be obtained from the patient (Figure 1).

Assessment of Immunotherapy Efficacy

After one year on a maintenance dose, clinical improvement should be apparent.²⁴ The therapy often may be discontinued after three to five years because by then the disease course has been altered.¹¹ Evaluation by an allergist, at least annually, should include monitoring of adverse reactions, assessment of efficacy, reinforcement of compliance and safe administration of immunotherapy, and determination of whether adjustments in the dosing schedule or allergen content are necessary.