Antibodies, also known as immunoglobulins, are specialized proteins produced by B cells as part of the adaptive immune response. Their primary function is to identify and neutralize antigens—molecular structures found on pathogens like viruses, bacteria, or toxins. By binding to these targets with high precision, antibodies initiate a coordinated response to clear the threat from the body.
The Basic Building Blocks
An antibody molecule is a glycoprotein that adopts a characteristic flexible Y-shape. This structure is composed of four polypeptide chains: two identical heavy chains and two identical, shorter light chains. Disulfide bonds connect these chains, creating a stable structure.
The molecule is functionally divided into two distinct regions. The two arms of the “Y” contain the variable regions, which include the antigen-binding site. This region has a diverse amino acid sequence that allows it to recognize and bind to a single, specific molecular shape on an antigen. The stem of the “Y” is the constant region, which determines the antibody’s class and general effector function.
The Mechanisms of Defense
Once an antibody binds to an antigen, the constant region activates several defense mechanisms. One primary action is neutralization, where the bound antibodies physically block the pathogen or toxin from interacting with host cells. For instance, by covering a virus’s surface proteins, antibodies prevent it from attaching to and entering a cell, halting the infection process.
Antibodies also facilitate opsonization, tagging the invader for destruction by phagocytic cells like macrophages and neutrophils. The constant region acts as a handle for these immune cells to attach to via specialized Fc receptors, enhancing the rate at which phagocytes engulf the foreign material.
A third defense strategy is agglutination, which involves antibodies cross-linking multiple pathogens into large clumps. Since each antibody has two antigen-binding sites, it can bind to antigens on two different cells, making the aggregated mass easier to clear. Furthermore, certain antibody classes can activate the complement system, a cascade of plasma proteins that punctures and lyses the pathogen’s cell wall.
The Antibody Family Five Classes and Their Roles
The five major classes, or isotypes, of antibodies are distinguished by differences in their heavy chain constant regions, which dictate their location and specialized roles throughout the body.
- Immunoglobulin G (IgG) is the most abundant class in the blood and tissue fluids, accounting for about 75% of all circulating antibodies. Its long half-life provides sustained protection, and it is the only antibody class capable of crossing the placenta to confer passive immunity to a developing fetus.
- Immunoglobulin A (IgA) is the dominant antibody found in mucosal secretions, such as saliva, tears, breast milk, and the lining of the respiratory and gastrointestinal tracts. It often exists as a dimer, which helps it defend against pathogens attempting to enter the body through mucosal surfaces.
- Immunoglobulin M (IgM) is structurally a large pentamer, composed of five Y-shaped units, and is the first antibody produced in response to a new infection. Its size and high valency make it effective at agglutination and activating the complement cascade early in the immune response.
- Immunoglobulin E (IgE) is present at low concentrations in the serum but is relevant in defense against parasitic worms and in allergic reactions. IgE binds to receptors on mast cells and basophils, triggering the release of inflammatory mediators upon encountering an allergen.
- Immunoglobulin D (IgD) is found mainly on the surface of B cells, where it functions as a receptor to signal the B cell’s activation and prompt the production of other antibody classes.