A Guide to Antibodies

Antibodies are uniquely Y-shaped and consist of four polypeptides or amino acid chains. They have two heavy chains and two light chains, each with many domains. The three main regions of an antibody allow it to perform different functions. Two antigen-binding fragments (Fab) make up the top of the Y, while a crystallizable fragment (Fc) forms its trunk.

The Fab is the section of an antibody that binds to antigens — foreign substances such as bacteria or viruses. Both tips of the Fab have a control and variable domain of a heavy and light chain. Within the variable domain are hypervariable domains that form the antigen-binding site or paratope.

An antibody’s trunk is the Fc region. Unlike the Fab, the Fc region consists exclusively of constant domains from the heavy chains. One of its functions is modulating immune cell activity. After the Fab binds to an antigen, the Fc region binds to an effector cell and triggers an effect. Additionally, the Fc region can dictate antibody distribution, such as sending antibodies across the placenta, from a mother to a fetus.

Antigens usually have different epitopes along their surfaces. These epitopes are like keys that are specific to an antibody’s lock or paratope. This allows the two regions to bind with extreme precision. Binding enables an antibody to tag the antigen for later destruction or neutralize it directly. Additionally, binding forces are weak and non-specific, meaning that the binding is reversible.

Initially, antibodies exist on the surfaces of B cells and are known as B-cell receptors. Once these bind to an antigen, the B cell activates and multiplies. Some of these new cells are plasma cells that create soluble antibodies specific to that antigen that spread to the blood and tissue fluids, as well as fluids like breast milk. Others become memory B cells and enable long-term immunity to the antigen.

Five isotypes or classes of antibodies exist. These classes function in different locations, manage different antigens, and have varying biological properties.

• IgA: Exists in mucosal regions, prevents pathogen colonization
• IgD: Unclear function, helps with B cell development, mostly binds to B cells
• IgE: Binds to allergens, triggers histamine releases, protects against parasites
• IgG: Provides most antibody-based immunity against pathogens, capable of crossing the placenta
• IgM: First response antibody, expressed on surfaces of B cells, eliminates pathogens before there is sufficient IgG to do so

Antibodies can perform many actions as part of their immune response.

• Neutralization: antibodies block the surface of antigens to hinder their attack
• Agglutination: antibodies “glue” foreign cells into clumps for phagocytosis or engulfing larger particles
• Precipitation: antibodies “glue” serum-soluble antigens together for phagocytosis
• Complement Activation or Fixation: antibodies latch onto a cell, attracting the first component of the complement cascade

Indirect actions an antibody can take include downregulating immune cells to prevent autoimmunity and presenting antibody fragments to T cells.

In rare cases, the immune system produces unique antibodies that attack the body’s own cells and tissues. These autoantibodies are responsible for autoimmune diseases. Rather than ignoring them like regular antibodies would, autoantibodies misidentify normal cells and bind to their epitopes. This causes the immune system to view those cells as foreign invaders and attack them. Experts do not know what causes autoantibodies to form.

Doctors will often use antibody tests to diagnose diseases. If antibodies for disease do not exist in a body, that person never had the disease or had it so long ago that their B cells have naturally decayed. Additionally, if the test results indicate that antibodies are binding to the body’s own epitopes, the individual likely has an autoimmune disease. Even higher levels of certain classes of antibodies may indicate specific diseases.

Scientists have recently begun producing synthetic antibodies with a wide range of applications. People who cannot create antibodies can receive supplements of these synthetic antibodies, allowing their immune systems to operate effectively. Synthetic antibodies may lower the risk of heart disease, prevent widespread infections, improve cancer screening, and even detect toxins in food.