germ-free mice Mice that are raised in the complete absence of intestinal and other microorganisms. Such mice have very depleted immune systems, but they can respond virtually normally to any specific antigen, provided it is mixed with a strong adjuvant.
germinal center Sites of intense B-cell proliferation and differentiation that develop in lymphoid follicles during an adaptive immune response. Somatic hypermutation and class switching occur in germinal centers.
germline theory An excluded hypothesis that antibody diversity was encoded by a separate germline gene for each antibody, known not to be true for most vertebrates, although cartilaginous fishes do have some rearranged V regions in the germline.
glycosylphosphatidylinositol (GPI) tail A glycolipid modification of proteins that can allow attachment to host membranes without the requirement of a transmembrane protein domain.
gnathostomes The class of jawed vertebrates comprising most fish and all mammals. These possess an adaptive immunity based on the RAG-mediated V(D)J recombination.
gnotobiotic mice See germ-free mice.
goblet cells Specialized epithelial cells located in many sites throughout the body responsible for mucus production; important in protection of the epithelium.
Goodpasture’s syndrome An autoimmune disease in which autoantibodies against type IV collagen (found in basement membranes) are produced, causing extensive inﬂammation in kidneys and lungs.
gout   Disease caused by monosodium urate crystals deposited in the cartilaginous tissues of joints, causing inﬂammation. Urate crystals activate the NLRP3 inﬂammasome, which induces inﬂammatory cytokines.
G proteins Intracellular GTPases that act as molecular switches in signaling pathways. They bind GTP to induce their active conformation, which is lost when GTO is hydrolyzed to GDP. There are two kinds of G proteins: the heterotrimeric (α, β, γ subunits) receptor-associated G proteins, and the small G proteins, such as Ras and Raf, which act downstream of many transmembrane signaling events.
G-protein-coupled receptors (GPCRs) A large class of seven-span transmembrane cell-surface receptors that associate with intracellular heterotrimeric G proteins after ligand binding, and signal by activation of the G protein. Important examples are the chemokine receptors.
G-quadruplex A structure formed from G-rich regions of DNA in which four guanine bases form a planar hydrogen-bonded network, or guanine tetrad, that can further stack on other guanine tetrads. G-quadruplexes processed from intronic switch region RNA may target AID back to the switch regions during isotype switching.
graft rejection See allograft rejection.
graft-versus-host disease (GVHD) An attack on the tissues of the recipient by mature T cells in a bone marrow graft from a nonidentical donor, which can cause a variety of symptoms; sometimes these are severe.
graft-versus-leukemia effect A beneficial side-effect of bone marrow grafts given to treat leukemia, in which mature T cells in the graft recognize minor histocompatibility antigens or tumor-specific antigens on the recipient’s leukemic cells and attack them.
Gram-negative bacteria   Bacteria that fail to retain crystal violet stain following alcohol wash due to a thin peptidoglycan layer.
Gram-negative binding proteins (GNBPs) Proteins that act as the pathogen-recognition proteins in the Toll pathway of immune defense in Drosophila.
granulocyte-macrophage stimulating factor (GM-CSF) A cytokine involved in the growth and differentiation of cells of the myeloid lineage, including dendritic cells, monocytes and tissue macrophages, and granulocytes.