Definition:

There are three major categories of antibody deficiencies: (a) defects in early B cell development, (b) hyper-IgM syndromes (also called class switch recombination defects), and (c) common variable immunodeficiency (CVID). Category (b), hyper-IgM syndrome (HIM), represents a group of distinct entities characterized by defective normal or elevated IgM in the presence of diminished IgG and IgA levels. Seventy per cent of the cases are X-linked in inheritance, and others are autosomal recessive. In the autosomal recessive hyper IgM syndromes, the problem lies in the nucleotide-editing enzymes AICD or UNG. These enzymes are only present in the germinal center B cells, and defects in either disrupt B-cell development and antibody production. Patients with these syndromes typically have recurrent bacterial infections and often have lymphoid hyperplasia.