Antigen
Also, an antigen is a molecule that binds to Ag-specific receptors, but cannot necessarily induce an immune response in the body by itself. Antigens are usually proteins, peptides (amino acid chains) and polysaccharides (chains of monosaccharides/simple sugars) but lipids and nucleic acids become antigens only when combined with proteins and polysaccharides. In general, saccharides and lipids (as opposed to peptides) qualify as antigens but not as immunogens since they cannot elicit an immune response on their own. Furthermore, for a peptide to induce an immune response (activation of T-cells by antigen-presenting cells) it must be a large enough size, since peptides too small will also not elicit an immune response.
Anti-neutrophil cytoplasmic antibody
Microbial superantigens are molecules expressed by bacteria and other microorganisms that have the power to stimulate a strong immune response by activation of T-cells. These molecules generally have regions that resemble self-antigens that promote a residual autoimmune response – this is the theory of molecular mimicry. Staphylococcal and streptococcal superantigens have been characterised in autoimmune diseases – the classical example in post group A streptococcal rheumatic heart disease, where there is similarity between M proteins of "Streptococcus pyogenes" to cardiac myosin and laminin. It has also been shown that up to 70% of patients with granulomatosis with polyangiitis (previously known as Wegener's granulomatosis) are chronic nasal carriers of "Staphylococcus aureus", with carriers having an eight times increased risk of relapse. This would therefore be considered a type II hypersensitivity reaction.