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

Test your knowledge of immunology fundamentals, including antigens, epitopes, and antibodies. Learn about the principles of immunohistochemistry and their importance in the immune response.