Antibodies

Antibodies are a type of protein called immunoglobulins, used in the body’s immune response. Antibodies are specific to complementary antigens on the cell surface membrane of non-self materials in the body, such as pathogens, toxins, and pollen.

Antibodies bind to these antigens, preparing cells and particles for destruction via phagocytosis.

How are antibodies produced?

Antibodies are produced by white blood cells called B lymphocytes (or B cells).

There are millions of different types of B cells in the body. Each cell produces one specific type of antibody, producing clones of identical antibodies. These are known as monoclonal antibodies because they are all the same.

The structure of an antibody

Antibodies are globular glycoproteins; they have a quaternary structure made up of four chains. Two of the chains are longer and called heavy chains. Two are shorter and called light chains.

Antibodies have a constant and variable region. The variable region is named that because it varies depending on the specific complementary antigen it binds with, while the lower constant region stays the same. The variable region includes the ends of both the light and heavy chains.

The antigen-binding site is found at the end of the variable region. The constant region binds to receptors on cells such as B cells. The four chains are connected by disulfide bridges.

Fig. 1 - Structure of an antibody

The importance of antibody structure

The structure of an antibody is important in shaping how antibodies function. Arguably, the most important part of any antibody is the antigen-binding site. This binding site has a highly specific 3-D structure. Because antibodies are proteins, they are able to form very specific shapes, which allows them to bind specifically only to complementary antigens. This means that antibodies can be specifically targeted at certain types of cells and pathogens.

Fig. 2 - Specific antigens binding to their complementary sites on the antibody

The characteristic 'Y' shape is also important. As you will see later, the structure of two binding sites instead of one allows antibodies to bind to two different antigens at once. This is important because it allows antibodies to effectively 'clump' together pathogens and other particles.

The antigen-antibody complex

Antibodies bind to antigens via the specific binding sites found on the ends of the variable region. Each binding site is made up of a sequence of amino acids that form a specific 3-D shape. This shape is complementary to a specific antigen. When an antibody and antigen bind together they form what is known as an antigen-antibody complex.

Fig. 3 - The antigen-antibody complex

How do antibodies work?

Antibodies do not destroy pathogens and their antigens directly. Instead, they prepare antigens for destruction by phagocytes. Phagocytes are white blood cells that engulf and digest pathogens as part of the immune response. Antibodies make it easier for cell-destroying defense mechanisms to find and destroy pathogens. Antibodies have two main methods of doing this:

Agglutination

One method is agglutination.

Antibodies can bind to two different pathogen antigens at once because antibodies have two binding sites. By clumping together multiple pathogens, antibodies make it easier for phagocytes to engulf and destroy pathogens in phagocytosis as all the antigens are in one centralized place.

Fig. 4 - The agglutination of antigens

Marking

Following agglutination, antibodies can act as chemical markers that stimulate phagocytes to move towards the clump of antibodies and antigens. Phagocytes can then engulf the clump of pathogens and destroy them in phagocytosis.

Antibodies and toxins

Antibodies have another role in the immune system, apart from binding to antigens. Antibodies are also able to bind to toxins produced by certain pathogens. When the toxin-antibody complex is formed, the toxin is neutralised, meaning that it cannot cause harm to the body. Following this, the toxin-antibody complex can be removed by phagocytes via phagocytosis.

Monoclonal antibodies are a specific, single type of antibody.

Monoclonal antibodies are becoming increasingly useful in science and medicine. They can be used to diagnose and treat different conditions and diseases.

Monoclonal antibodies are also useful in diagnosing different diseases. Monoclonal antibodies are used in the ELISA test, which is used to diagnose a broad range of diseases, including HIV. A step-by-step overview of the ELISA test and its use of monoclonal antibodies can be found in our article on HIV.