Antigen
In immunology, antigens (Ag) are structures (aka substances) specifically bound by antibodies (Ab) or a cell surface version of Ab ~ B cell antigen receptor (BCR). The term antigen originally described a structural molecule that binds specifically to an antibody only in the form of native antigen. It was expanded later to refer to any molecule or a linear molecular fragment after processing the native antigen that can be recognized by T-cell receptor (TCR). BCR and TCR are both highly variable antigen receptors diversified by somatic V(D)J recombination. Both T cells and B cells are cellular components of adaptive immunity. The Ag abbreviation stands for an antibody generator.
Anti-neutrophil cytoplasmic antibody
The c-ANCA antigen is specifically proteinase 3 (PR3). p-ANCA antigens include myeloperoxidase (MPO) and bacterial permeability increasing factor (BPI). Other antigens exist for c-ANCA (atypical), however many are as yet unknown. Classical p-ANCA occurs with antibodies directed to MPO. p-ANCA without nuclear extension occurs with antibodies to BPI, cathepsin G, elastase, lactoferrin and lysozyme. GS-ANA are antibodies directed to granulocyte specific nuclear antigens. Atypical ANCA are thought to be antigens similar to that of the p-ANCAs, however may occur due to differences in neutrophil processing.