In addition to spontaneous hypothyroidism and autoimmune thyroiditis, chickens of the obese strain (OS) have a high incidence of selective IgA deficiency. Elevated levels of serum IgM also occur in many OS chickens. The IgA deficiency begins by 2 weeks, the age when hypothyroidism is developing. Like thyroiditis, a greater incidence of IgA deficiency was noted in OS birds homozygous for the B1 allele than for the B4 allele of the major histocompatibility locus. Although IgA deficiency occurs in both sexes, it is found in a higher frequency in females than males (2:1). An abnormal ontogenesis of immunological competence may influence both traits.
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