Antibodies: Function & Diseases

Antibodies, also known as immunoglobulins, play a central role in the human immune system. These macromolecules circulate in the blood and mediate the humoral immune response of all higher vertebrates.

What are antibodies?

Plasma cells are cells of the immune system and are used to produce and form antibodies. Orange: plasma cells, white: antibodies. Click to enlarge. Antibodies are proteins found in the blood, on immune cells, and in extracellular tissue fluid. Their production is triggered by an antigen (for antibody-generating). Antigens are usually foreign substances, for example surface structures on pathogens such as bacteria or viruses. Antibodies recognize and bind to these antigens, whereupon the pathogen is neutralized and eliminated. They are released into the blood by a specific class of white blood cells called plasma cells. Plasma cells are differentiated B lymphocytes. B lymphocytes, in turn, are a specific class of white blood cells. Each B lymphocyte recognizes a specific antigen. Upon contact with “its” antigen, the B lymphocyte is activated and begins producing antibodies directed against that same antigen.

Medical and health functions, roles, and meanings.

When antibodies make contact with their antigen, they trigger a humoral immune response against it. Their three main functions in doing so are neutralization, opsonization, and activation of the complement system. They accomplish all this by binding their antigen. An antibody is a large molecule with a ypsilon-shaped structure. The stem of the ypsilon and the lower part of the two short arms belong to the so-called constant domain. It is identical in all antibodies of the same class or isotype. The variable domains are located at the ends of the two short arms of the ypsilon. They form the specific antigen-binding sites that recognize a very specific “epitope” on the surface of an antigen. An epitope is a submolecular structure, for example a short section of a surface protein of a bacterium (the surface protein would then be the antigen). Thanks to the two arms, each antibody can bind two of “its” epitopes and thereby also cross-link multiple antigens, which is called agglutination. Neutralization: Antibodies neutralize toxins, bacteria and viruses by binding to them and preventing them from entering human body cells. Opsonization: Once an antibody has bound its antigen, it marks it for other immune cells, such as phagocytes, which then eliminate the antigen. Complement system: This is a cascade of over thirty proteins that successively bind to the surface of microorganisms (e.g., a bacterium) and trigger multiple immune mechanisms. They can mark the bacterium for phagocytes, trigger inflammatory responses, or lead directly to lysis by driving pores into the cell membrane. An antibody bound to the surface of a bacterium can activate the complement system via the so-called “classical pathway.”

Diseases, ailments, and disorders

An elevated titer of antibodies in the blood indicate an ongoing immune response and, therefore, an infection. Also, the antibodies present tell whether someone has been vaccinated against certain diseases. The antibodies themselves mediate the vaccination protection. In passive immunization, the patient is directly injected with antibodies directed against a specific pathogen. This vaccine protection does not last long because the injected antibodies break down over time and cannot be replicated. In active immunization, antigens rather than antibodies are injected. These can be attenuated or killed pathogens or parts of pathogens (purified surface molecules of viruses and bacteria). The immune system of the vaccinated person then recognizes epitopes on the injected antigens and produces antibodies against them. If the vaccinated person later comes into contact with the pathogens, the antibodies already present immediately trigger an immune response. The pathogens are eliminated before they can cause disease. Some vaccines (for example, against the childhood diseases measles, mumps and rubella) can confer lifelong immunity. As a rule, therefore, active immunization is preferable to passive immunization, provided that the vaccine is safe.Impaired antibody production (e.g. due to inherited B-cell defects) triggers various immunodeficiency diseases. When antibodies bind to the body’s own epitopes and subsequently trigger an immune response, autoimmune diseases occur.