typeIV

Type IV is a designation used in several scientific fields to label a category identified by the Roman numeral IV. The term appears most often in biology and medicine, where it refers to distinct, unrelated concepts that share the same numeric label.

Type IV collagen refers to a family of network-forming basement membrane collagens. Encoded mainly by COL4A genes, these chains assemble into heterotrimers that form a meshwork rather than fibrils. This network provides structural support in tissues such as the kidney, eye, and cochlea. Mutations in type IV collagen genes can cause hereditary nephropathies, most notably Alport syndrome, and can be associated with ocular and auditory abnormalities.

Type IV secretion systems are complex protein assemblies used by many Gram-negative bacteria to move proteins—and sometimes DNA—across cellular membranes into host cells or the extracellular environment. These systems resemble a conjugation apparatus and are powered by ATPases. They play a central role in virulence for several pathogens and have been harnessed as tools in plant and mammalian genetic engineering, notably via Agrobacterium tumefaciens.

Type IV pili are thin, retractable filaments on the surfaces of many bacteria. They mediate adhesion to surfaces and other cells, enable twitching motility, and facilitate genetic transformation by taking up DNA from the environment. Their dynamics are driven by dedicated ATPases and assembly/retraction machinery.

Type IV hypersensitivity, or delayed-type hypersensitivity, is an immunological reaction driven primarily by T lymphocytes and macrophages rather than antibodies. It typically develops 24 to 72 hours after antigen exposure and underlies clinical tests such as the tuberculin skin test and contact dermatitis. Cytokines such as IFN-γ coordinate the inflammatory response in affected tissues.