Review Question - QID 217220

This patient who presents with recurrent systemic infections and is found to have decreased expression of a dimer protein on peripheral mononuclear cells most likely has major histocompatibility complex (MHC) class II deficiency. These patients will have normal B cells, decreased CD4 cells, and normal CD8 cells.

MHC class II proteins are heterodimers that are expressed exclusively on antigen-presenting cells such as peripheral mononuclear cells. They are produced with an invariant chain that prevents loading of antigen peptides until the MHC class II proteins arrive in endolysosomal compartments. There, they bind to endocytosed fragments and present these antigens to CD4+ T-cells in order to stimulate the adaptive immune system. Patients with defective MHC class II function will therefore have decreased abundance of CD4+ helper T-cells and be susceptible to a broad array of infectious agents since helper T-cells are important in anti-bacterial, anti-viral, and anti-parasitic defenses.

Hanna and Etzioni present a review of the clinical and pathophysiological manifestations of MHC I and MHC II deficiencies. They emphasize the functional importance of these proteins.

Incorrect Answers:
Answer 1: Decreased B cells, decreased CD4 cells, and decreased CD8 cells would be seen in patients with severe combined immunodeficiency disorder. This disease is caused predominantly by a defect in adenosine deaminase and would not affect the abundance of MHC proteins on peripheral mononuclear cells. This disease presents with life-threatening infections since birth.

Answer 2: Decreased B cells, normal CD4 cells, and normal CD8 cells would be seen in patients with Bruton agammaglobulinemia. This disease is caused by a defect in the Bruton tyrosine kinase that promotes B-cell development and survival. This disease is found almost exclusively in boys and presents with recurrent soft tissue and pulmonary infections.

Answer 3: Normal B cells, decreased CD4 cells, and decreased CD8 cells would be seen in patients with a defect in T-cell survival factor production such as IL-2 deficiency. These disorders are extremely rare and would not affect the abundance of MHC proteins on peripheral mononuclear cells.

Answer 5: Normal B cells, normal CD4 cells, and decreased CD8 cells would be seen in patients with MHC class I deficiency. MHC class I has a single long protein chain bound to beta-2-microglobulin rather than 2 chains of relatively equal size. This disease presents with recurrent viral infections.

Bullet Summary:
MHC class II proteins are expressed on peripheral mononuclear cells and are important in stimulating and promoting the development of CD4+ T cells.