42.12: Antibodies - Antibody Structure

Learning Objectives

• Differentiate among the classes of antibodies

An antibody is a molecule that recognizes a specific antigen; this recognition is a vital component of the adaptive immune response. Antibodies are composed of four polypeptides: two identical heavy chains (large peptide units) that are partially bound to each other in a “Y” formation, which are flanked by two identical light chains (small peptide units). The area where the antigen is recognized on the antibody is known as the variable domain or variable region. This is why there are numerous antibodies that can each recognize a different antigen. The antibody base is known as the constant domain or constant region. The portion of an antigen that is recognized by the antibody is known as the epitope.

Figure \(\PageIndex{1}\): Antibodies: (a) As a germ-line B cell matures, an enzyme called DNA recombinase randomly excises V and J segments from the light chain gene. Splicing at the mRNA level results in further gene rearrangement. As a result, (b) each antibody has a unique variable region capable of binding a different antigen.

Antibody variation

In B cells, the variable region of the light chain gene has 40 variable (V) and five joining (J) segments. An enzyme called DNA recombinase randomly excises most of these segments out of the gene, splicing one V segment to one J segment. During RNA processing, all but one V and J segment are spliced out. Recombination and splicing may result in over 10⁶ possible VJ combinations. As a result, each differentiated B cell in the human body typically has a unique variable chain. The constant domain, which does not bind to an antibody, is the same for all antibodies. The large diversity of antibody structure translates into the large diversity of antigens that antibodies can bind and recognize.

Similar to TCRs (T cell receptors) and BCRs (B cell receptors), antibody diversity is produced by the mutation and recombination of approximately 300 different gene segments encoding the light and heavy chain variable domains in precursor cells that are destined to become B cells. The variable domains from the heavy and light chains interact to form the binding site through which an antibody can bind a specific epitope on an antigen. The numbers of repeated constant domains in Ig classes (discussed below) are the same for all antibodies corresponding to a specific class. Antibodies are structurally similar to the extracellular component of the BCRs. The maturation of B cells into plasma cells occurs when the cells gain the ability to secrete the antibody portion of its BCR in large quantities.

Antibody Classes

Antibodies can be divided into five classes (IgM, IgG, IgA, IgD, and IgE) based on their physiochemical, structural, and immunological properties. Ig stands for immunoglobulin, another term for an antibody. IgGs, which make up about 80 percent of all antibodies in circulation, have heavy chains that consist of one variable domain and three identical constant domains. IgA and IgD also have three constant domains per heavy chain, whereas IgM and IgE each have four constant domains per heavy chain. The variable domain determines binding specificity, while the constant domain of the heavy chain determines the immunological mechanism of action of the corresponding antibody class. It is possible for two antibodies to have the same binding specificities, but be in different classes and, therefore, to be involved in different functions.

After an adaptive defense is produced against a pathogen, typically plasma cells first secrete IgM into the blood. BCRs on naïve B cells are of the IgM class and, occasionally, the IgD class. IgM molecules comprise approximately ten percent of all antibodies. Prior to antibody secretion, plasma cells assemble IgM molecules into pentamers (five individual antibodies) linked by a joining (J) chain. The pentamer arrangement means that these macromolecules can bind ten identical antigens. However, IgM molecules released early in the adaptive immune response do not bind to antigens as stably as do IgGs, which are one of the possible types of antibodies secreted in large quantities upon re-exposure to the same pathogen. The properties of immunoglobulins and their basic structures are shown in the table.

Figure \(\PageIndex{1}\): Classes of antibodies: Immunoglobulins (antibody classes) have different functions, but all are composed of light and heavy chains that form a Y-shaped structure.

IgAs populate the saliva, tears, breast milk, and mucus secretions of the gastrointestinal, respiratory, and genitourinary tracts. Collectively, these bodily fluids coat and protect the extensive mucosa (4000 square feet in humans). The total number of IgA molecules in these bodily secretions is greater than the number of IgG molecules in the blood serum. A small amount of IgA is also secreted into the serum in monomeric form. Conversely, some IgM is secreted into bodily fluids of the mucosa. Similarly to IgM, IgA molecules are secreted as polymeric structures linked with a J chain. However, IgAs are secreted mostly as dimeric molecules, not pentamers.

IgE is present in the serum in small quantities and is best characterized in its role as an allergy mediator. IgD is also present in small quantities. Similarly to IgM, BCRs containing the IgD class of antibodies are found on the surface of naïve B cells. This class supports antigen recognition and subsequent maturation of B cells to plasma cells.

Key Points

• Antibodies contain four polypeptides: two identical (to each other) heavy chains in a “Y” formation and two idenitical (to each other) light chains on the outside of the top of the “Y” portion.
• Each antibody has a unique variable region, which is responsible for antigen detection and specificity.
• There are five classes of antibodies, each utilized by the body under different conditions, including IgM, IgG, IgA, IgD, and IgE; Ig stands for immunoglobulin.
• IgAs, secreted in the milk, tears and mucous, are the most numerous antibodies produced; inside of the body, circulating IgGs are the most abundant.

Key Terms

• immunoglobulin: any of the glycoproteins in blood serum that respond to invasion by foreign antigens and that protect the host by removing pathogens; also known as an antibody
• antigen: a substance that binds to a specific antibody; may cause an immune response
• B cell: a lymphocyte, developed in the bursa of birds and the bone marrow of other animals, that produces antibodies and is responsible for the immune system
• epitope: that part of a biomolecule (such as a protein) that is the target of an immune response; the part of the antigen recognized by the immune system