Antibodies are protein molecules made by mature B cells called plasma cells. Antibodies, also called immunoglobulins, come in several types, including immunoglobulin A (IgA), IgD, IgE, IgG and IgM. Each type of antibody has a different role within the immune system. The importance of antibodies in protecting the body from infection is highlighted by the fact that people with low or defective antibodies suffer from recurrent infections of the ears, sinuses and lungs.
Parts of an Antibody
Plasma cells are the cells that produce antibodies. On the end where the two points are, the antibody has an area called the variable region, which is able to detect molecular signals on infectious organisms such as bacteria and viruses. The single end of the Y shape, or constant region, is specially made to attach to other proteins and cells of the immune system.
How Antibodies Work
Antibodies help to stop infections in many distinct ways. For viruses, the antibodies might attach to regions on the virus that the virus uses to dock to cells, blocking the virus from infecting the cell. Antibodies can perform a similar neutralizing function with toxins released by bacteria, preventing those chemicals from causing damage to cells. Antibodies also can attach to bacteria and then attract and activate complement, immune system molecules that punch holes in the surface of the bacteria, causing death. Finally, antibodies are able to attach to other cells such as mast cells, eosinophils, macrophages and neutrophils, stimulating those cells to kill the organism attached to the other end of the antibody.
Types of Antibodies
Immunoglobulin M, or IgM is the initial type of antibody made by B cells, the precursors to plasma cells. IgM molecules can combine in groups of five to form pentamers. These pentamers work together to detect repeating patterns on the surface of bacteria. Next, the cells make IgD. The function of IgD is not fully understood. If stimulated properly, plasma cells can then shift to make IgG, IgA or IgE. IgA is unusual because it is able to cross from the blood to the mucosal surfaces of the body like the intestines and oral cavity and the vaginal lining in women. IgE is important in parasitic infections and is also involved in allergic reactions. IgG, which differentiates further into several subtypes, is the most abundant antibody type. It has the ability to cross the placenta, and babies use transferred IgG from their mothers to fight infection during the first six months of life.
Antibody Deficiency
Some people are born with decreased levels of antibodies or defective antibodies. The most common type of immunodeficiency is IgA deficiency, where the plasma cells are unable to switch to making IgA. Hyper IgM disease happens when plasma cells having difficulty shifting from IgM to any of the other subtypes. More severe symptoms and infection occur with common variable immunodeficiency or CVID, where plasma cells do not develop properly, and all antibody types are low. Many other genetic antibody immunodeficiencies exist. People with certain types of leukemias and lymphomas can also have difficulty producing effective antibodies.
Symptoms of Antibody Deficiency
While antibodies do a lot of jobs, for many infections the immune system would be able to compensate and protect the body even if antibodies were not there. The main exception to this is for certain types of bacteria such as Streptococcus pneumoniae and Hemophilus influenza, which have sugars coating the cell membrane. Not having antibodies can also mean that the body is less likely to remember an infection it had in the past. Finally, not having normal antibodies can make the immune system body more likely to overreact. For example, people with CVID and hyper IgM are more likely to have autoimmune diseases and allergies are more common in people with IgA deficiency.
References
- "Immunobiology, 5th Edition: The Immune System in Health and Disease"; Chapter 4 The Generation of Lymphocyte Antigen Receptors; Charles A Janeway, Jr, Paul Travers, Mark Walport, and Mark J Shlomchik; 2001
- "Clinical Immunology Principles and Practice"; Primary Antibody Deficiencies; Harry W. Schroeder; 2008
- "Clinical Immunology Principles and Practice"; Immunoglobulin Function; Neil S. Greenspan; 2008
