Failure to thrive (FTT) refers to a child whose physical growth is substantially below the norm.

Abnormally increased frequency (usually defined as three or more) loose or watery bowel movements a day.

An increased susceptibility to bronchitis as manifested by a history of recurrent bronchitis.

An increased susceptibility to lower respiratory tract infections as manifested by a history of recurrent lower respiratory tract infections.

A lasting absence of total IgG and total IgA and total IgM in the blood circulation, whereby at most trace quantities can be measured.

Increased susceptibility to otitis media, as manifested by recurrent episodes of otitis media.

An abnormally low number of neutrophils in the peripheral blood.

Increased susceptibility to bacterial infections, as manifested by recurrent episodes of bacterial infection.

An abnormality of T cells.

Complete lack of mature B cells, that is, of B cells that have left the bone marrow.

Absent or undetectable level of isohemagglutinin. An isohemagglutinin refers to the naturally occurring antibodies in the ABO blood group system (i.e., anti-A in a group B person, anti-B in a group A person, and anti-A, anti-B, and anti-A,B in a group O person).