The inflammatory infiltrate in oral LP is primarily composed of CD8+ T cells. A potential pathway for CD8+ T cell-mediated cytotoxicity in oral LP is described as follows:Antigens presented on MHC 1 molecules activates CD8+ T cells on keratinocytes or by encounters with activated CD4+ helper T cells or cytokines produced by activated CD4+ helper T cells
Activated CD8+ T cells induce keratinocyte apoptosis through various mechanisms such as secretion of tumor necrosis factor (TNF)-alpha, secretion of granzyme B, or Fas-Fas ligand interactions. Chemokines are produced by activated CD8+ T cells that attract additional inflammatory cells, thereby promoting continued inflammation. Other mechanisms that have been proposed include:

upregulation of matrix metalloproteinases that disrupt the epithelial basement membrane zone and allow entry of immune cells into the epidermis,
the release of proinflammatory mediators and proteases by mast cells, and
perturbations in the innate immune response that may involve toll-like receptors.Oral LP may also be caused by genetic factor which influence the immune function. A separate study performed in China found an association between a polymorphism in the TNF-alpha gene and risk for oral LP in a subset of patients. An Italian study found a significant increase in a genetic polymorphism of the first intron of the interferon (IFN)-gamma promoter in patients with oral LP compared with controls. Diagnosis
Skin
Patient history and clinical presentation need to be taken to diagnose lichen planus. Patients with suspected cutaneous lichen planus need to be evaluated clinically through patient interviews and physical examinations.
PID or Pid may refer to:

Medicine
Pelvic inflammatory disease or pelvic inflammatory disorder, an infection of the upper part of the female reproductive system
Primary immune deficiency, disorders in which part of the bodys immune system is missing or does not function properly
Prolapsed intervertebral disc, commonly called a herniated disc

Science, technology and engineering
BBC Programme Identifier, a unique identifier for a BBC television or radio programme brand, a season or series, or an individual episode
OBD-II PIDs (on-board diagnostics parameter IDs), requests for data through an OBD connector in automotive repair
Packet Identifier, a field in a MPEG transport stream packet
Passive infrared detector, a passive infrared sensor
Persistent identifier, a long-lasting reference to a document, file, web page, or other object
Phosphotyrosine-interacting domain, also known as phosphotyrosine-binding domain, a protein domain which bind to phosphotyrosine
Photoionization detector, measures volatile organic compounds and other gases
Physical Interface Device, a class of a USB device
PID controller (proportional-integral-derivative controller), a control concept used in automation
Piping and instrumentation diagram (P&ID), a diagram in the process industry which shows the piping of the process flow etc.