One of the body’s most important defense mechanisms against infection is the production by the reticuloendothelial tissues of a class of proteins called antibodies or immunoglobulin’s.
These proteins circulate in the bloodstream, where they make up part of the “gamma globulin” fraction of blood plasma.
The production of the immunoglobulin’s is stimulated by chemical substances, called antigens, present in or released from the infecting agent (e.g., constituents of bacterial membranes or the coats of viruses) and recognized as being foreign or alien to the body.
This response is called the “immune response.” Although antigens can belong to virtually any chemical category, they usually are proteins, polysaccharides, or nucleic acids.
The immunoglobulin molecules that are produced during an immune response have the capacity to bind to the antigens, the reaction between the two being highly specific; that is, each type of immunoglobulin that is produced in the body reacts with a particular antigen and no other. A major source of the antibodies that circulate in the blood are the “plasma cells” derived by the differentiation of a class of white blood cells known as B lymphocytes.
The binding of antibodies to antigens that are in the surface of invading microorganisms (such as bacteria) triggers a series of reactions that leads to the destruction of the foreign cells. These reactions involve a host of other blood plasma proteins collectively referred to as complement and that are normally present in the plasma (i.e., unlike the immunoglobulin’s, they are not synthesized in response to the appearance of an antigen).
Attachment of the immunoglobulin molecules to the surface antigens of the infecting cells is followed by the sequential binding and activation of the complement proteins. Acting in a manner similar to digestive enzymes, the complement proteins create holes in the bacterial surface. This produces a “Donnan effect” in which ions and water enter the foreign cell, causing it to swell and eventually rupture (i.e., lyse).
In effect, the immunoglobulin’s by binding to the surface antigens have served to identify the cells that are to be attacked and destroyed by complement. Each antibody has more than one antigen- binding site. Therefore, when free antigen molecules are encountered in the body, they can be cross-linked to form a large precipitable complex.
When antibodies react simultaneously with antigens in the surfaces of separate cells, the cells are agglutinated, that is, they are clumped together into small masses. Precipitated antigens and agglutinated cells are ultimately engulfed and disposed of by phagocytic white blood cells. The immune response appears to be of rather recent evolutionary origin, because immunoglobulin production is characteristic only of vertebrates.
Immunoglobulin Structure:
The human body is capable of synthesizing more than a million different kinds of immunoglobulin molecules, each capable of reacting with a different antigen, but all of them appear to share the same fundamental quaternary structure. During the early stages of an infection, the response to the antigen involves the production of a specific class of immunoglobulin’s known as immunoglobulin M or IgM, having a molecular weight of about 1,000,000. Later, the amount of IgM gradually declines as another class or “isotype” of immunoglobulin called IgG appears.
Although other immunoglobulin’s are also present in blood (see Table 4-11), IgG represents the most abundant form (as much as 80%). IgG has been more extensively studied than the other immunoglobulin’s and much of the description that follows is based on results obtained from studies of IgG.
IgG molecules with a molecular weight of about 150,000 are composed of four polypeptide chains of two different kinds—a pair of identical high- molecular-weight chains called “heavy” or H chains and a pair of identical lower-molecular-weight chains called “light” or L chains. The L chains have a molecular weight of 20,000 to 25,000 and consist of about 214 amino acids. The H chains have a molecular weight of 50,000 to 55,000 and contain about 450 amino acids. In associating with each other, the four chains form a molecule whose quaternary structure resembles that of the letter “Y” (Fig. 4-35).
Each arm of the Y contains a complete L chain and part of an H chain and the leg of the Y contains the remaining parts of the H chains. Near its C-terminus, each L chain is linked to an H chain by a disulfide bridge, and two additional disulfide bridges link the H chains together.Each of the four polypeptide chains that form an immunoglobulin is divided into separate regions called “domains.” There are two domains in the L chains and four in the H chains.
Within each of the domains, folding of the polypeptide chain produces two parallel planes each containing several segments with folded beta structure. In each folded beta structure, neighboring stretches of the polypeptide have opposite polarity (i.e., they are antiparallel). The two parallel planes in each domain are held together by disulfide bridges and also by van der Waals interactions between hydrophobic side chains of .amino acids in each sheet.
In each of the arms of the immunoglobulin, the H and L chains associate to establish a globular quaternary structure maintained principally by van der Waals interactions between hydrophobic side chains of amino acids in each chain; some hydrogen bonding between chains also occurs.
Antigen- and Complement-Binding Sites:
Studies employing digestive enzymes such as trypsin and papain have revealed which portions of an immunoglobulin molecule combine with the antigens and which part binds complement. These digestive enzymes characteristically cleave immunoglobulin’s in a region (called the “hinge” region) located on the C- terminal side of the disulfide bridge that links each L chain to an H chain (see Fig. 4-35).
Two types of fragments are produced by such enzymatic cleavage, one of which retains the capacity to bind the antigen (called the Fab fragment). Each of the two Fab fragments that is produced consists of a complete L chain and the N-terminal half of an H chain. The other type of fragment consists of the C-terminal halves of both H chains and is called the Fc fragment because it can be crystallized. The antigen appears to be bound near the ends of the two Fab units (the shaded area in Fig. 4-35).
The angle between the Fab and Fc units is variable because of the flexibility of the hinge regions of the molecule. As a result, the overall shape of an immunoglobulin can vary between that of the letter “Y” and the letter “T.”
This flexibility is unquestionably related to the immunoglobulin’s capacity to bind to the antigens. The complement proteins associate with the Fc fragment. Immunoglobulin’s contain a small amount of carbohydrate (i.e., the immunoglobulin’s are glycoproteins). The carbohydrate portion consists of a short chain of sugars covalently bonded to an asparagine residue and sandwiched along the interface between the Fab and Fc units.