Immunology Basics

Antigen and Epitope

• Antigen: a molecule that induces an immune response and has one or more antibody binding sites
• Antigens can be found in bacteria, viruses, and tumors
• Epitope: a specific topographical region composed of a small number of amino acids or monosaccharide units

Antibodies

• Serum proteins known as immunoglobulins, found in blood and tissue fluids, as well as many secretions
• Antibodies can be monomeric, dimeric, trimeric, tetrameric, or pentameric
• Monomer composed of two heavy and two light chains
• Cleaved with enzymes such as papain and pepsin, producing two Fab (fragment antigen-binding) and Fc (fragment crystallizable)
• Formed in the humoral immune system by plasma cells, the end cell of B-lymphocyte transformation after recognition of a foreign antigen

Types of Antibodies

• IgA, IgD, IgE, IgG, and IgM
• IgG is the most common and frequently used antibody for immunohistochemistry
• IgG molecule composed of two pairs of light and heavy polypeptide chains linked by disulfide bonds, forming a Y-shaped structure
• Terminal regions of each arm vary in amino acid sequence and are known as ‘variable domains’

Affinity and Avidity

• Affinity: the three-dimensional fit of the antibody to its specific antigen, measuring the binding strength between the antigenic epitope and its specific antibody-combining site
• Avidity: a related property referring to the heterogeneity of the antiserum, containing various antibodies reacting with different epitopes of the antigen molecule
• Avidity is the strength of multiple interactions between a multivalent antigen and antibody

Antibody Specificity and Sensitivity

• Antibody specificity: the characteristic of an antibody to bind selectively to a single epitope on an antigen
• Antibody sensitivity: the relative amount of antigen that an immunohistochemical technique is able to detect