Bare lymphocyte syndrome is a condition caused by mutations in certain genes of the major histocompatibility complex or involved with the processing and presentation of MHC molecules.

[2] In BLS II the immune system is severely compromised and cannot effectively fight infection due to an inability for antigen presenting cells to activate CD4⁺ t-cells as no TCR recognition of MHC II/peptide complexes can occur.

Clinically, this is similar to severe combined immunodeficiency (SCID), in which lymphocyte precursor cells are improperly formed.

The genes responsible were cloned by the laboratories of Bernard Mach[6] in Switzerland and Jeremy Boss[7] at Emory University in Atlanta, Georgia.

[8] TAP (Transporter associated with antigen processing) proteins are involved in pumping degraded cytosolic peptides across the endoplasmic reticulum membrane so they can bind to HLA class I.