Monarch Disease Ontology identifier MONDO_0008855 (MHC class II deficiency) can be described as follows. Immunodeficiency by defective expression of HLA class 2 is a rare primary genetic immunodeficiency disorder characterized by partial or complete absence of human leukocyte antigen class 2 expression resulting in severe defect in both cellular and humoral immune response to antigens. The disorder presents clinically as marked susceptibility to infections, severe malabsorption and failure to thrive and is often fatal in early childhood. Also known as: HLA class 2-negative SCID, HLA class 2-negative severe combined immunodeficiency, MHC class II expression deficiency, SCID due to absent class II HLA antigens, immunodeficiency by defective expression of HLA class type 2, major histocompatibility complex class II expression deficiency.