Lesson Video: Structure and Function of Antibodies Biology

Video Transcript

In this video, we will learn to distinguish between an antigen and antibody and describe the structure of an antibody. We will then learn about the mechanisms used by antibodies to limit the effects of pathogens and toxins.

Imagine that you had the superpower of electron microscope vision. Using your superpower, you would be able to see that nearly every surface, including your own skin, is covered in microscopic substances, such as viruses, fungi, and bacteria. The vast majority of these microbes are harmless or even beneficial, but a small number have the potential to cause disease. We call this type of organism or biological agent a pathogen. Thankfully, our immune system defends us against non-self-substances, which includes not only pathogens but allergens and foreign objects, such as splinters. It can even defend us against self-substances that may be harmful, such as cancer cells.

When the immune system detects a threat, either a non-self-substance or a potentially harmful self-substance, there are two types of pathways through which it can respond: the nonspecific response, sometimes called the innate response, or the specific response, sometimes called the adaptive response. The nonspecific response is used to quickly defend against nearly all types of threats. Some examples of nonspecific responses are physical barriers, such as the skin, or the inflammatory response. The specific response is triggered when the threat is an antigen. The initial or primary specific response can be quite slow, taking up to two weeks. But it produces special B cells, or B lymphocytes, called memory cells. Memory cells earn their name by creating an immunological memory of a particular antigen. So, if they encounter the same antigen at a later date, the secondary response is fast and powerful.

An example of a specific response is called humoral immunity. Although there are no shortage of doctor jokes, in medicine, the term “humor” refers to body fluids, not to comedy. In the humoral response, proteins called antibodies, also known as immunoglobulins, target antigens and the extracellular body fluids, including plasma, lymph, and saliva. An antibody is a globular protein produced by B lymphocytes. Antibodies bind to specific antigens as the first step in the humoral response.

Now let’s take a closer look at these two types of molecules, starting with antigens. Antigens are found on the surface of nearly all cells, but a healthy immune system typically does not respond to its own antigens. These are known as self-antigens, and they include the antigens found on red blood cells, which determine our blood type. Instead, a healthy immune system responds to non-self-antigens, including those found on the surface of pathogens or foreign tissues. Allergens, such as pollen or egg whites, can also act as non-self-antigens, as can toxins, which are harmful substances produced by some living organisms.

The term “antigen” is actually a shortened version of the phrase “antibody generator,” because when an antigen is recognized by an immune cell, it triggers the production of antibodies. Antibodies can be found on the surface of B cells, where they’re often referred to as immunoglobulins, or circulating in body fluids, including plasma, lymph, and saliva. An individual B cell can have up to 100,000 antibodies on its surface. But all of the antibodies on a particular B cell will be identical, meaning they can only bind to a single specific antigen, a feature known as complementary binding. Because each of the roughly one trillion B cells in the human body produces a unique antibody, it allows our immune system to respond to an astounding diversity of antigens.

When a surface immunoglobulin on a B cell recognizes a specific antigen, the B cell is activated to undergo multiple rounds of mitosis, resulting in a large number of cells called plasma cells. It is these plasma cells that act as the antibody generators, producing and secreting antibodies that are complementary to the antigen that first activated the response. These secreted antibodies will then circulate in extracellular fluid in order to target the same antigen.

Now let’s take a closer look at the structure of an antibody to see how it achieves complementary binding with a specific antigen. Antibodies are Y-shaped proteins, with three distinct globular or globe-shaped regions. This globular shape provides an example of the key biological concept that structure determines function. The globular shape keeps the hydrophobic, or not-water-soluble, portions facing towards the center of the globe while keeping the hydrophilic, or water-soluble, portions facing towards the outside. This structure allows antibodies to be water-soluble, which in turn informs their function, to circulate freely in body fluids to fight infection.

The basic structure of an antibody consists of four polypeptide chains: two identical heavy chains, shown in orange, and two identical light chains, shown in pink. The chains are held together by disulfide bonds, shown in green, and this forms the characteristic Y shape. Both the heavy and light chains have a constant region, shown in the areas with pink and orange dots. The constant regions are composed of identical amino acid sequences. Each chain also has a variable region, shown in the area with pink and orange stripes.

The variable regions are composed of different amino acid sequences. At the end of each variable region is an antigen binding site. And it is the unique structure of this antigen binding site that determines which antigen the antibody will be able to bind to. The two antigen binding sites on a given antibody are identical, meaning that each antibody can bind to two identical antigens simultaneously. At the base of the antibody is a receptor binding site, which allows an antibody–antigen complex to bind to a variety of immune cells. Before we go into more detail on the function of antibodies, it’s important to note that they do not directly destroy non-self-materials or potentially harmful self-materials, such as cancer cells. Instead, they serve to limit the harmful effects of these substances while facilitating other immune system processes.

One way the antibodies do this is through opsonization. Opsonins are proteins that bind to the surface of particles or cells, and antibodies are opsonins that bind to pathogens. When many antibodies bind to the surface of a pathogen, it creates a sort of chemical tag, marking the pathogen for destruction through other immune processes. This occurs because the formation of an antigen–antibody complex on the surface of a pathogen makes the receptor binding sites on antibodies more accessible to other immune cells, for example, macrophages, which have antibody receptors on their surface.

Opsonization with a macrophage leads to a process called phagocytosis. Phago- is a Greek prefix that means to eat, and cyto- refers to a cell, which tells us that phagocytosis occurs when a cell eats another cell or particle by first engulfing it in its plasma membrane and then fusing the resulting vesicle with organelles called lysosomes. Lysosomes are organelles that contain digestive enzymes. When the lysosomes release these enzymes, shown here as small green dots, they degrade or break down the pathogen inside the vesicle. The degraded bits of the pathogen can then be discharged from the cell as waste product.

In addition to tagging a pathogen or foreign substance for destruction, the formation of an antibody–antigen complex can block the binding sites on a pathogen, thereby preventing it from entering a host cell. This is referred to as neutralization. And it is another way the antibodies work to limit the harmful effects of pathogens. In order for a virus to enter a host cell, it must first bind to a receptor on its surface. The virus is then taken into the cell and forms a structure called an endosome. Inside the endosome, the viral and host membranes fuse, which allows the release of the virus’s genetic material into the host cell. However, if a neutralizing antibody binds to the virus, it blocks the virus from binding to the host cell receptor. In some cases, the virus may still be able to enter the host cell. But the presence of neutralizing antibodies blocks the fusion of the host and viral membranes, thereby preventing the release of the virus’s genetic material. Once a virus has been neutralized by an antibody, it will eventually be tagged for phagocytosis.

Neutralization can also occur through special antibodies called antitoxins. You may remember that toxins are harmful substances produced by some living organisms. Toxins can enter the body directly through inhalation, ingestion, injection, or absorption through the skin or mucous membranes. Toxins can also be produced by a pathogen after it has entered a host. When toxins produced by a pathogen bind to toxin receptors on a host cell membrane, they can enter the cell, damaging it or even causing death. But if antitoxins first bind to the toxins, it blocks them from binding to the host cell receptors, thereby preventing cell damage or death.

Antitoxins are of medical importance because they can be produced commercially for the treatment of diseases caused by toxins. For example, spores of the bacteria Clostridium botulinum produce a powerful neurotoxin. If food containing these spores is consumed, the neurotoxin can lead to the disease botulism. If untreated, botulism can lead to respiratory failure, paralysis, and even death. The antitoxin for botulinum can be used to prevent or treat botulism. It is also stockpiled by the governments of some countries, in case botulinum is used in a bioterrorism attack.

Agglutination describes how pathogens carrying antibody–antigen complexes can clump together. The formation of clumps is possible because an antibody has two identical antigen binding sites, allowing it to bind to the same antigen on two separate pathogens and cross-linking the pathogens together. Although all types of antibodies are capable of cross-linking pathogens into clumps, some types are particularly effective. For example, a type of antibody called immunoglobulin M, or IgM for short, naturally forms pentamers or groups of five. The resulting 10 antigen binding sites on each pentamer means that IgM can easily cross-link many pathogens into large clumps. The formation of clumps slows the spread of the pathogen through the body and speeds up phagocytosis by allowing macrophages to engulf several pathogens at once.

Agglutination can be used to quickly determine blood type by mixing the patient’s blood with anti-A antibodies and anti-B antibodies. Anti-A antibodies bind to A antigens, which are found on the surface of some red blood cells, while anti-B antibodies bind to B antigens. After adding the patient’s blood to both tubes, we can see that there was no agglutination in the anti-A tube, while agglutination did occur in the anti-B tube, indicating that the patient’s red blood cells only produce the B antigens, and their blood type is B. Because antibodies are soluble, they’re typically found in a solution and are not visible. Some antigens are also soluble. However, under the right conditions, antibody–antigen complexes can cross-link to form a lattice-like structure that is insoluble. We call this insoluble visible product a precipitate, and the process of forming a precipitate through complementary binding of antigens and antibodies is known as precipitation.

Precipitation is similar to agglutination, but the antigens involved are soluble and are typically larger, while agglutination involves insoluble antigens that are typically smaller. For precipitation to occur, the ratio of antigen to antibody must fall within a certain range, called the zone of equivalence. In the zone of equivalence, the ratio of antigen to antibody is approximately one to one. And as shown on the graph, this is where maximum precipitation will occur. Excess antibody or excess antigen prevents efficient cross-linking, resulting in little to no precipitation.

Certain antibodies can also defend against pathogens through the use of lysis. This occurs when antibody–antigen complexes activate the complement system, which is part of the nonspecific immune response. When the complement system is activated, complement proteins bind to the receptor sites of antibody–antigen complexes. The binding of complement proteins in turn activates the formation of membrane attack proteins, which insert themselves into the cell membranes or envelopes of pathogens, creating holes called pores. Water flows through the pore into the pathogen, causing it to swell and eventually killing the pathogen through lysis.

Now that we’ve thoroughly discussed antibody structure and function, let’s try a practice question.

The figure represents the structure of an antibody. Where does an antigen bind?

To identify where an antigen binds on an antibody, let’s first look at its various components. An antibody, sometimes called an immunoglobulin, is a soluble protein made up of four polypeptide chains. There are two identical longer chains, known as heavy chains, that make up the central portion of the antibody and two identical shorter chains on the exterior, known as the light chains. The chains are held together by disulfide bonds, one of which is labeled with the letter A. The disulfide bonds hold the chains together in a characteristic Y shape. And at the base of this Y is a receptor binding site, which allows the antibody to bind to surface receptors on cells, particularly other immune system cells, such as macrophages. At the top of each branch of the Y is an antigen binding site. The two antigen binding sites bind to a single specific antigen based on their shape, much like a key fits into a specific lock. Because there are two identical antigen binding sites, a single antibody can bind to two identical antigens simultaneously. Therefore, on this diagram, the sites where antigens can bind are B and C.

Now let’s review and summarize some of the key points from the video. Antigens are molecules that trigger a specific immune response. An example of a specific response is humoral immunity, which produces antibodies. Antibodies are soluble proteins composed of four polypeptide chains: two identical heavy chains and two identical light chains. Antibodies have a characteristic Y shape. And at the tips of each branch of the Y are the variable regions, which allow binding to a single specific antigen. Antibodies do not directly destroy antigens or the pathogens that produce antigens. Instead, they use processes such as opsonization, agglutination, and precipitation to facilitate phagocytosis. Antibodies can also neutralize pathogens or toxins to limit their harmful effects or form complexes that activate the complement system, leading to lysis of pathogens.