Allergen

Cow's milk is popularly consumed throughout the world. Allergy to cow's milk is prevalent among infants and young children during the first year of life, affecting 0.5-3% at the age of 1 year. Bos d 8 (casein) has been reported as a major allergenic protein in cow's milk. According to a number of studies, sensitization towards Bos d 8 ranges between 0.9-65% of patients with cow's milk allergy. Bos d 8 has been found to be a highly heat-resistant protein, and its allergenicity is found to remain despite heating at 120oC (for 15 min). Therefore, specific immunoglobulin E (sIgE) levels of Bos d 8 may help assess the likelihood of tolerance to cooked or baked milk. Further, low sIgE levels to Bos d 8 have been reported to indicate tolerance towards cow's milk. Clinical manifestations of Bos d 8-mediated allergy majorly include atopic dermatitis. Moreover, Bos d 8 sIgE in serum has been regarded as a beneficial tool for predicting resolution and management with respect to introducing baked milk products into the diet. Cross-reactivity has been observed between Bos d 8 (casein) from cow's milk and casein from goat, sheep, and human milk.

Epidemiology

Worldwide distribution

Cow's milk is a common food consumed worldwide by infants, children, and adults. It is considered one of the major food allergens prevalent among infants and young children, affecting 0.5-3% with cow's milk allergy (CMA) at the age of 1 year, with a high prevalence observed during the first year of life. However, the majority of the children outgrow their CMA by the age of 5 years and develop tolerance.

Bos d 8 (casein) has been reported as a major allergenic protein in cow's milk. According to the studies reported in the literature, the prevalence of sensitization toward casein is varied (range 0.9%-65%) among patients with CMA, which might be due to the diverse study population.

A US-based study was conducted on 78 children (7.9-14.4 years) with moderate to severe atopic dermatitis (AD). The study reported Bos d 8 to be responsible for generating the highest specific immunoglobulin E (sIgE) level among milk-allergic children.

A study was conducted in Japan on 83 children (0.8-15.8 years) with suspected CMA. The CMA children (n=61; clinically diagnosed) were claimed to possess a considerably higher level of Bos d 8 sIgE compared to the non-CMA children.

A study in the Netherlands included 28 adults (≥16 years) CMA patients [18 assessed via skin prick test (SPT) and 10 via double-blind placebo-controlled food challenge] in the study group and 25 individuals in the control group (cow milk sensitized but tolerant individuals). The study detected an elevated level (p=0.016) of Bos d 8 sIgE in individuals present in the study group compared to the control group.

Clinical Relevance

Disease severity

CMA is considered one of the most common pediatric food allergies. Milk allergy can be either IgE-mediated or non-IgE-mediated. The clinical manifestations of cow milk protein allergy are reported to be of two types, such as immediate (shortly after consumption) and delayed (after hours or days of consumption) responses. The immediate reactions are generally IgE-mediated and can lead to angioedema, urticaria, aggravation of AD, vomiting, and in some cases, anaphylaxis, as well as respiratory symptoms (breathing, wheezing, coughing). On the other hand, the majority of the delayed reactions are non-IgE mediated and can lead to gastrointestinal symptoms (like enteropathy or proctocolitis) or skin reactions (such as AD).

Bos d 8 has been found to be a highly heat-resistant protein among all other milk proteins. The allergenicity of Bos d 8 has been found to remain despite heating at 120oC (for 15 min).

Therefore, sIgE level to Bos d 8 may help assess the likelihood of tolerance to cooked or baked milk, and 75% of children with CMA may actually tolerate baked milk products such as muffins, waffles, cakes and breads. High sIgE levels of Bos d 8, however, indicate an allergy to all forms of milk, and low sIgE levels to Bos d 8 indicate the patient may develop tolerance.

This heat-resistant property of Bos d 8 has been validated by a study conducted on 134 CMA children (subjected to oral food challenges to various baked milk products). The study demonstrated significantly lower Bos d 8 sIgE among the baked-milk tolerant group compared to the baked-milk reactive group.

A prospective, observational, real-life study was conducted on 100 clinically diagnosed CMA children (79 with AD and 29 without AD). Children with AD showed cutaneous reactions as the symptom of CMA. The study reported that the children affected with AD exhibited an elevated level of sIgE towards Bos d 8, particularly at 6 months (age).

Cross-reactive molecules

Milk of different ruminant species (cow, goat, buffalo, and sheep) has been reported to possess Bos d 8 proteins bearing high sequence homology (80 to >90%), which could probably be the reason for cross-reactivity between these mammalian milk. A prospective cohort study on 38 children (<14 years) with CMA-associated symptoms detected the rate of cross-sensitization (SPT-based) between goat milk and cow milk to be 63.2% (24/38).

A broad dissimilarity (less Bos d 8 and more whey protein) in protein distribution has been observed in donkey and horse (mare) compared to cow's milk, which has been believed to be responsible for the low cross-reactivity of these milk with cow's milk. Similarly, a lower molecular similarity between cow and camel milk has been identified as responsible for a weak cross-reactivity between these two types of milk. A prospective cohort study identified cross-sensitization between camel and cow milk in 20% (7/38) of children with CMA.

Moreover, in vitro cross-reactivity between cow milk Bos d 8 and soy protein A5-B3 glycinin molecule (11 S globulin of soybean) has been reported.

Diagnostics

Disease Severity

Milk-sIgE testing is used as an aid in diagnosing milk allergy and monitoring disease development. A direct correlation has been observed between elevated levels of Bos d 8 sIgE and persistent CMA. In contrast, lower levels of Bos d 8 sIgE are found to be associated with higher chances of tolerance towards baked milk products. Hence, Bos d 8 sIgE in serum is regarded as a beneficial tool for predicting resolution and management with respect to introducing baked milk products into the diet. The cut-off decision points (combined with clinical history and physician expertise) for sIgE to Bos d 8 can be used by physicians to identify the ideal candidates for baked milk OFCs and improve the management of children with suspected CM allergy. As per a US-based study, the positive and negative decision points for reactivity to baked milk were 20.2 kUA/L and approximately < 0.94 kUA/L, respectively, for Bos d 8 sIgE.

The presence of an increased level of sIgE to Bos d 8 has been reported to be correlated with milk allergy among children and may be linked with persistent CMA. According to a study, children with persistent CMA have been found to possess significantly higher ratios of sIgEs to linear vs conformational epitopes to α- and β-casein (p<0.005 and p<0.002, respectively) compared to the individuals outgrowing milk allergy. A high concentration of sIgE towards Bos d 8 might be a prognostic factor of a reaction to baked milk in children with CMA. Measurement of κ-casein has been suggested as the most beneficial marker to predict reactivity to baked milk and allergies.

Exposure

The route of exposure for the Bos d 8 component found in milk is through ingestion.

However, direct skin contact (cutaneous exposure) mediated anaphylaxis in a 12-month-old child post-exposure to Bos d 8 milk allergen-containing ointment in diaper rash has been reported in a case study.

Also, inhalation-mediated allergic reactions (asthma and rhinitis) toward milk protein Bos d 8 (present in a dermatological powder) were observed in a 44-year-old nurse with a positive SPT towards Bos d 8.

Explained Results

Allergen information

Bos d 8 (casein) is a major allergenic protein in cow's milk. It is a heat-resistant protein sensitive to enzymatic digestion in the gut.

Clinical relevance

Typical symptoms of CMA include skin reactions (angioedema, urticaria, and atopic dermatitis), GI symptoms, respiratory systems, and in some cases, anaphylaxis.

Cross-reactivity

Bos d 8 (casein) from cow's milk has shown extensive cross-reactivity with casein from goat, sheep, and human milk. Low cross-reactivity has been reported between bovine milk and soy, donkey, horse (mare), and camel milk.