Transporter associated with antigen processing (TAP) deficiency syndrome, the most well-characterized condition among the type I bare lymphocyte syndromes (BLS), is a rare autosomal recessive primary immunodeficiency disorder due to a defect within the TAP complex. The TAP complex, a heterodimer consisting of 2 subunits (TAP1 and TAP2) synthesized from genes located on the human leukocyte antigen (HLA) locus of chromosome 6, is involved in the processing and subsequent presentation of antigenic peptides via cell-surface major histocompatibility complex (MHC) class I molecules during the immune response. Deletion or mutation of the genes encoding either subunit of the TAP complex results in impaired antigenic processing and down-regulation of MHC class I expression on the surface of lymphocytes. The resulting impaired immune response leads to increased susceptibility to bacterial infections.

Patients with TAP deficiency syndrome typically present with recurrent bacterial infections of the upper respiratory tract (chronic purulent rhinitis, sinusitis, otitis media) within the first 6 years of life. This is followed by recurrent bacterial infections of the lower respiratory tract (bacterial pneumonia, chronic bronchitis, and subsequent bronchiectasis) and skin (necrotizing granulomatous lesions) within the second decade of life. Common culprits include both gram positive (Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae) and gram negative (Klebsiella, Escherichia coli, Pseudomonas aeruginosa) organisms. Early age of onset and a positive family history can be additional diagnostic clues.

If left untreated, recurrent respiratory infections with subsequent bronchiectasis and progressive respiratory failure may significantly increase morbidity and mortality. Septicemia is an additional complication.