immunology allergic disorders
Hyper IgM Syndrome
X linked hyper IgM syndrome
Most cases are X-linked and caused by mutations in a gene on the X chromosome that encodes a protein (CD154, or CD40 ligand) on the surfaces of activated helper T cells. In the presence of cytokines, normal CD40 ligand interacts with B cells and thus signals them to switch from producing IgM to producing IgA, IgG, or IgE. In X-linked hyper-IgM syndrome, T cells lack functional CD40 ligand and cannot signal B cells to switch. Thus, B cells produce only IgM; IgM levels may be normal or elevated. Patients with the X-linked form may have severe neutropenia and often present during infancy with Pneumocystis jirovecii Pneumonia . Lymphoid tissue is very small because deficient CD 40 ligand signaling does not activate B cells. Otherwise, clinical presentation is similar to that of 