heavy-chain variable region (VH)   Referring to the V region of the heavy chain of an immunoglobulin.
helicard See MDA-5.
helper CD4 T cells, helper T cells   Effector CD4 T cells that stimulate or ‘help’ B cells to make antibody in response to antigenic challenge. TH2, TH1, and the TFH subsets of effector CD4 T cells can perform this function.
hemagglutinin (HA)   Substances that can cause hemagglutination, such as human antibodies that recognize the ABO blood group antigens on red blood cells, or the inﬂuenza virus hemagglutinin, a glycoprotein that functions in viral fusion with endosome membranes.
hematopoietic stem cells (HSCs) Type of pluripotent cell in the bone marrow that can give rise to all the different blood cell types.
hematopoietin superfamily Large family of structurally related cytokines that includes growth factors and many interleukins with roles in both adaptive and innate immunity.
hemochromatosis protein A protein expressed by intestinal epithelial cells that regulates iron uptake and transport by interacting with the transferrin receptor to decrease its affinity for iron-loaded transferrin.
hemolytic disease of the newborn A severe form of Rh hemolytic disease in which maternal anti-Rh antibody enters the fetus and produces a hemolytic anemia so severe that the fetus has mainly immature erythroblasts in the peripheral blood.
hemophagocytic lymphohistiocytic (HLH) syndrome A dysregulated expansion of CD8-positive lymphocytes that is associated with macrophage activation. The activated macrophages phagocytose blood cells, including erythrocytes and leukocytes.
hepatobiliary route Route whereby mucosally produced dimeric IgA enters the portal veins in the lamina propria, is transported to the liver, and reaches the bile duct by transcytosis. This pathway is not of great significance in humans.
heptamer The conserved seven-nucleotide DNA sequence in the recombination signal sequences (RSSs) ﬂanking gene segments in the immunoglobulin and T-cell receptor loci.
HER-2/neu A receptor tyrosine kinase overexpressed in many cancers, particularly breast cancer, that is the target of trastuzumab (Herceptin) used in its treatment.
herd immunity Protection conferred to unvaccinated individuals in a population produced by vaccination of others and reduction in the natural reservoir for infection.
hereditary angioedema (HAE) A genetic deficiency of the C1 inhibitor of the complement system. In the absence of C1 inhibitor, spontaneous activation of the complement system can cause diffuse ﬂuid leakage from blood vessels, the most serious consequence of which is swelling of the larynx, leading to suffocation.
hereditary hemochromatosis A disease caused by defects in the HFE gene characterized by abnormally high retention of iron in the liver and other organs.
herpes virus entry molecule (HVEM) See B and T lymphocyte attenuator.
heterosubtypic immunity Immune protection against a pathogen conferred by infection with a distinct strain, typically with reference to different inﬂuenza A serotypes.
heterotrimeric G proteins See G proteins.
heterozygous Describes individuals that have two different alleles of a given gene, one inherited from the mother and one from the father.
HFE See hemochromatosis protein.
high endothelial cells, high endothelial venules (HEV) Specialized small venous blood vessels in lymphoid tissues. Lymphocytes migrate from the blood into lymphoid tissues by attaching to the high endothelial cells in the walls of the venules and squeezing between them.