The distribution and functions of immunoglobulin isotypes (2024)

9-12. Antibodies of different isotype operate in distinct places and have distincteffector functions

Pathogens most commonly enter the body across the epithelial barriers of themucosa lining the respiratory, digestive, and urogenital tracts, or throughdamaged skin, and can then establish infections in the tissues. Less often,insects, wounds, or hypodermic needles introduce microorganisms directly intothe blood. The body's mucosal surfaces, tissues, and blood are all protected byantibodies from such infections; these antibodies serve to neutralize thepathogen or promote its elimination before it can establish a significantinfection. Antibodies of different isotypes are adapted to function in differentcompartments of the body. Because a given V region can become associated withany C region through isotype switching (see Section 4-16), the progeny of a single B cell can produceantibodies, all specific for the same eliciting antigen, that provide all of theprotective functions appropriate for each body compartment.

The first antibodies to be produced in a humoral immune response are always IgM,because IgM can be expressed without isotype switching (see Figs 4.20 and 9.8). These early IgM antibodies are produced before B cells haveundergone somatic hypermutation and therefore tend to be of low affinity. IgMmolecules, however, form pentamers whose 10 antigen-binding sites can bindsimultaneously to multivalent antigens such as bacterial capsularpolysaccharides. This compensates for the relatively low affinity of the IgMmonomers by multipoint binding that confers high overall avidity. As a result ofthe large size of the pentamers, IgM is mainly found in the blood and, to alesser extent, the lymph. The pentameric structure of IgM makes it especiallyeffective in activating the complement system, as we will see in the last partof this chapter. Infection of the bloodstream has serious consequences unless itis controlled quickly, and the rapid production of IgM and its efficientactivation of the complement system are important in controlling suchinfections. Some IgM is also produced in secondary and subsequent responses, andafter somatic hypermutation, although other isotypes dominate the later phasesof the antibody response.

Antibodies of the other isotypes—IgG, IgA, and IgE—are smaller in size anddiffuse easily out of the blood into the tissues. Although IgA can form dimers,as we saw in Chapter 4, IgG and IgEare always monomeric. The affinity of the individual antigen-binding sites fortheir antigen is therefore critical for the effectiveness of these antibodies,and most of the B cells expressing these isotypes have been selected forincreased affinity of antigen-binding in germinal centers. IgG is the principalisotype in the blood and extracellular fluid, whereas IgA is the principalisotype in secretions, the most important being those of the mucus epithelium ofthe intestinal and respiratory tracts. Whereas IgG efficiently opsonizespathogens for engulfment by phagocytes and activates the complement system, IgAis a less potent opsonin and a weak activator of complement. This distinction isnot surprising, as IgG operates mainly in the body tissues, where accessorycells and molecules are available, whereas IgA operates mainly on epithelialsurfaces where complement and phagocytes are not normally present, and thereforefunctions chiefly as a neutralizing antibody.

Finally, IgE antibody is present only at very low levels in blood orextracellular fluid, but is bound avidly by receptors on mast cells that arefound just beneath the skin and mucosa, and along blood vessels in connectivetissue. Antigen binding to this IgE triggers mast cells to release powerfulchemical mediators that induce reactions, such as coughing, sneezing, andvomiting, that can expel infectious agents, as will be discussed below when wedescribe the receptors that bind immunoglobulin C regions and engage effectorfunctions. The distribution and main functions of antibodies of the differentisotypes are summarized in Fig.9.19.

9-13. Transport proteins that bind to the Fc regions of antibodies carry particularisotypes across epithelial barriers

IgA-secreting plasma cells are found predominantly in the connective tissuecalled the lamina propria, which lies immediately below the basem*nt membrane ofmany surface epithelia. From there, the IgA antibodies can be transported acrossthe epithelium to its external surface, for example, to the lumen of the gut orthe bronchi. IgA antibody synthesized in the lamina propria is secreted as adimeric IgA molecule associated with a single J chain (see Fig. 4.23). This polymeric form of IgA binds specificallyto the poly-Ig receptor, which is present on the basolateral surfaces of theoverlying epithelial cells (Fig. 9.20).When the poly-Ig receptor has bound a molecule of dimeric IgA, the complex isinternalized and carried through the cytoplasm of the epithelial cell in atransport vesicle to its luminal surface. This process is calledtranscytosis. At the apical or luminal surface of theepithelial cell, the poly-Ig receptor is cleaved enzymatically, releasing theextracellular portion of the receptor still attached to the Fc region of thedimeric IgA. This fragment of receptor, called the secretory component, may help to protect the IgA dimerfrom proteolytic cleavage. Some molecules of dimeric IgA diffuse from the laminapropria into the extracellular spaces of the tissues, draining into thebloodstream before being excreted into the gut via the bile. Therefore, it isnot surprising that patients with obstructive jaundice, a condition in whichbile is not excreted, show a marked increase in dimeric IgA in the plasma.

The principal sites of IgA synthesis and secretion are the gut, the respiratoryepithelium, the lactating breast, and various other exocrine glands such as thesalivary and tear glands. It is believed that the primary functional role of IgAantibodies is to protect epithelial surfaces from infectious agents, just as IgGantibodies protect the extracellular spaces of the internal tissues. IgAantibodies prevent the attachment of bacteria or toxins to epithelial cells andthe absorption of foreign substances, and provide the first line of defenseagainst a wide variety of pathogens. Newborn infants are especially vulnerableto infection, having had no prior exposure to the microbes in the environmentthey enter at birth. IgA antibodies are secreted in breast milk and are therebytransferred to the gut of the newborn infant, where they provide protection fromnewly encountered bacteria until the infant can synthesize its own protectiveantibody.

IgA is not the only protective antibody a mother passes on to her baby. MaternalIgG is transported across the placenta directly into the bloodstream of thefetus during intrauterine life; human babies at birth have as high a level ofplasma IgG as their mothers, and with the same range of antigen specificities.The selective transport of IgG from mother to fetus is due to an IgG transportprotein in the placenta, FcRn, which is closely related in structure to MHCclass I molecules. Despite this similarity, FcRn binds IgG quite differentlyfrom the binding of peptide to MHC class I, as its peptide-binding groove isoccluded. It binds to the Fc portion of IgG molecules (Fig. 9.21). Two molecules of FcRn bind one molecule ofIgG, bearing it across the placenta. In some rodents, FcRn also delivers IgG tothe circulation of the neonate from the gut lumen. Maternal IgG is ingested bythe newborn animal in its mother's milk and colostrum, the protein-rich fluidsecreted by the early postnatal mammary gland. In this case, FcRn transports theIgG from the lumen of the neonate gut into the blood and tissues. Interestingly,FcRn is also found in adults in the gut and liver and on endothelial cells. Itsfunction in adults is to regulate the levels of IgG in serum and other bodyfluids, which it does by binding circulating antibody, endocytosing it, and thenrecycling to the cell surface.

By means of these specialized transport systems, mammals are supplied from birthwith antibodies against pathogens common in their environments. As they matureand make their own antibodies of all isotypes, these are distributed selectivelyto different sites in the body (Fig.9.22). Thus, throughout life, isotype switching and the distributionof isotypes through the body provide effective protection against infection inextracellular spaces.

9-14. High-affinity IgG and IgA antibodies can neutralize bacterial toxins

Many bacteria cause disease by secreting proteins called toxins, which damage ordisrupt the function of the host's cells (Fig.9.23). To have an effect, a toxin must interact specifically with amolecule that serves as a receptor on the surface of the target cell. In manytoxins, the receptor-binding domain is on one polypeptide chain whereas thetoxic function is carried by a second chain. Antibodies that bind to thereceptor-binding site on the toxin molecule can prevent the toxin from bindingto the cell and thus protect the cell from attack (Fig. 9.24). Antibodies that act in this way to neutralizetoxins are referred to as neutralizing antibodies.

Most toxins are active at nanomolar concentrations: a single molecule ofdiphtheria toxin can kill a cell. To neutralize toxins, therefore, antibodiesmust be able to diffuse into the tissues and bind the toxin rapidly and withhigh affinity. The ability of IgG antibodies to diffuse easily throughout theextracellular fluid and their high affinity make these the principalneutralizing antibodies for toxins found in tissues. IgA antibodies similarlyneutralize toxins at the mucosal surfaces of the body.

Diphtheria and tetanus toxins are two bacterial toxins in which the toxic andreceptor-binding functions are on separate protein chains. It is thereforepossible to immunize individuals, usually as infants, with modified toxinmolecules in which the toxic chain has been denatured. These modified toxins,called toxoids, lack toxic activity but retain the receptor-binding site. Thus,immunization with the toxoid induces neutralizing antibodies that protectagainst the native toxin.

With some insect or animal venoms that are so toxic that a single exposure cancause severe tissue damage or death, the adaptive immune response is too slow tobe protective. Exposure to these venoms is a rare event and protective vaccineshave not been developed for use in humans. Instead, neutralizing antibodies aregenerated by immunizing other species, such as horses, with insect and snakevenoms to produce anti-venom antibodies (antivenins) for use in protectinghumans. Transfer of antibodies in this way is known as passive immunization (see Appendix I, Section A-37).

9-15. High-affinity IgG and IgA antibodies can inhibit the infectivity ofviruses

Animal viruses infect cells by binding to a particular cell-surface receptor,often a cell-type-specific protein that determines which cells they can infect.The hemagglutinin of influenzavirus, for example, binds to terminal sialic acid residues on the carbohydratesof glycoproteins present on epithelial cells of the respiratory tract. It isknown as hemagglutinin because it recognizes and binds to similar sialic acidresidues on chicken red blood cells and agglutinates these red blood cells.Antibodies to the hemagglutinin can prevent infection by the influenza virus.Such antibodies are called virus-neutralizing antibodies and, as with theneutralization of toxins, high-affinity IgA and IgG antibodies are particularlyimportant.

Many antibodies that neutralize viruses do so by directly blocking viral bindingto surface receptors (Fig. 9.25).However, viruses are sometimes successfully neutralized when only a singlemolecule of antibody is bound to a virus particle that has many receptor-bindingproteins on its surface. In these cases, the antibody must cause some change inthe virus that disrupts its structure and either prevents it from interactingwith its receptors or interferes with the fusion of the virus membrane with thecell surface after the virus has engaged its surface receptor.

9-16. Antibodies can block the adherence of bacteria to host cells

Many bacteria have cell-surface molecules called adhesins that enable them tobind to the surface of host cells. This adherence is critical to the ability ofthese bacteria to cause disease, whether they subsequently enter the cell, as dosome pathogens such as Salmonella species, or remain attachedto the cell surface as extracellular pathogens (Fig. 9.26). Neisseria gonorrhoeae, the causativeagent of the sexually transmitted disease gonorrhea, has a cellsurface proteinknown as pilin. Pilin enables the bacterium to adhere to the epithelial cells ofthe urinary and reproductive tracts and is essential to its infectivity.Antibodies against pilin can inhibit this adhesive reaction and preventinfection.

IgA antibodies secreted onto the mucosal surfaces of the intestinal, respiratory,and reproductive tracts are particularly important in preventing infection bypreventing the adhesion of bacteria, viruses, or other pathogens to theepithelial cells lining these surfaces. The adhesion of bacteria to cells withintissues can also contribute to pathogenesis, and IgG antibodies against adhesinscan protect from damage much as IgA antibodies protect at mucosal surfaces.

9-17. Antibody:antigen complexes activate the classical pathway of complement bybinding to C1q

Another way in which antibodies can protect against infection is by activation ofthe cascade of complement proteins. We have described these proteins in Chapter 2, as they can also beactivated on pathogen surfaces in the absence of antibody, as part of the innateimmune response. Complement activation proceeds via a series of proteolyticcleavage reactions, in which inactive components, present in plasma, are cleavedto form proteolytic enzymes that attach covalently to the pathogen surface. Allknown pathways of complement activation converge to generate the same set ofeffector actions: the pathogen surface or immune complex is coated withcovalently attached fragments (principally C3b) that act as opsonins to promoteuptake and removal by phagocytes. At the same time, small peptides withinflammatory and chemotactic activity are released (principally C5a) so thatphagocytes are recruited to the site. In addition, the terminal complementcomponents can form a membrane-attack complex that damages some bacteria.

Antibodies initiate complement activation by a pathway known as the classical pathway because it was the first pathway of complement activation to bediscovered. The full details of this pathway, and of the other two knownpathways of complement activation, are given in Chapter 2, but we will describe here how antibody is ableto initiate the classical pathway after binding to pathogen, or after formingimmune complexes.

The first component of the classical pathway of complement activation is C1,which is a complex of three proteins called C1q, C1r, and C1s. Two moleculeseach of C1r and C1s are bound to each molecule of C1q (see Fig. 2.10). Complement activation is initiated whenantibodies attached to the surface of a pathogen bind C1q. C1q can be bound byeither IgM or IgG antibodies but, because of the structural requirements ofbinding to C1q, neither of these antibody isotypes can activate complement insolution; the cascade is initiated only when the antibodies are bound tomultiple sites on a cell surface, normally that of a pathogen.

The C1q molecule has six globular heads joined to a common stem by long,filamentous domains that resemble collagen molecules; the whole C1q complex hasbeen likened to a bunch of six tulips held together by the stems. Each globularhead can bind to one Fc domain, and binding of two or more globular headsactivates the C1q molecule. In plasma, the pentameric IgM molecule has a planarconformation that does not bind C1q (Fig.9.27, left panel); however, binding to the surface of a pathogendeforms the IgM pentamer so that it looks like a staple (see Fig. 9.27, right panel), and thisdistortion exposes binding sites for the C1q heads. Although C1q binds with lowaffinity to some subclasses of IgG in solution, the binding energy required forC1q activation is achieved only when a single molecule of C1q can bind two ormore IgG molecules that are held within 30–40 nm of each other as a result ofbinding antigen. This requires many molecules of IgG to be bound to a singlepathogen. For this reason, IgM is much more efficient in activating complementthan is IgG. The binding of C1q to a single bound IgM molecule, or to two ormore bound IgG molecules, leads to the activation of an enzymatic activity inC1r, triggering the complement cascade as shown schematically in Fig. 9.28. This translates antibodybinding into the activation of the complement cascade, which, as we learned inChapter 2, can also betriggered by direct binding of C1q to the pathogen surface.

9-18. Complement receptors are important in the removal of immune complexes fromthe circulation

Many small soluble antigens form antibody:antigen complexes known as immunecomplexes that contain too few molecules of IgG to be readily boundto the Fcγ receptors we will discuss in the next part of the chapter. Theseantigens include toxins bound by neutralizing antibodies and debris from deadmicroorganisms. Such immune complexes are found after most infections and areremoved from the circulation through the action of complement. The solubleimmune complexes trigger their own removal by activating complement, againthrough the binding of C1q, leading to the covalent binding of the activatedcomponents C4b and C3b to the complex, which is then cleared from thecirculation by the binding of C4b and C3b to CR1 on the surface of erythrocytes.The erythrocytes transport the bound complexes of antigen, antibody, andcomplement to the liver and spleen. Here, macrophages bearing CR1 and Fc receptors remove the complexes from the erythrocyte surface without destroyingthe cell, and then degrade them (Fig.9.29). Even larger aggregates of particulate antigen and antibody canbe made soluble by activation of the classical complement pathway, and thenremoved by binding to complement receptors.

Immune complexes that are not removed tend to deposit in the basem*nt membranesof small blood vessels, most notably those of the renal glomerulus where theblood is filtered to form urine. Immune complexes that pass through the basem*ntmembrane of the glomerulus bind to the complement receptor CR1 on the renalpodocytes, cells that lie beneath the basem*nt membrane. The functionalsignificance of these receptors in the kidney is unknown; however, they play animportant part in the pathology of some autoimmune diseases.

In the autoimmune disease systemic lupus erythematosus (Systemic Lupus Erythematosus, inCase Studies in Immunology, see Preface for details), which we will describe inChapter 13, excessive levelsof circulating immune complexes cause huge deposits of antigen, antibody, andcomplement on the podocytes, damaging the glomerulus; kidney failure is theprincipal danger in this disease. Immune complexes can also be a cause ofpathology in patients with deficiencies in the early components of complement.Such patients do not clear immune complexes effectively and they also suffertissue damage, especially in the kidneys, in a similar way.

Summary

The T-cell dependent antibody response begins with IgM secretion but quicklyprogresses to the production of all the different isotypes. Each isotype isspecialized both in its localization in the body and in the functions it canperform. IgM antibodies are found mainly in blood; they are pentameric instructure. IgM is specialized to activate complement efficiently upon bindingantigen. IgG antibodies are usually of higher affinity and are found in bloodand in extracellular fluid, where they can neutralize toxins, viruses, andbacteria, opsonize them for phagocytosis, and activate the complement system.IgA antibodies are synthesized as monomers, which enter blood and extracellularfluids, or as dimeric molecules in the lamina propria of various epithelia. IgAdimers are selectively transported across these epithelia into sites such as thelumen of the gut, where they neutralize toxins and viruses and block the entryof bacteria across the intestinal epithelium. Most IgE antibody is bound to thesurface of mast cells that reside mainly just below body surfaces; antigenbinding to this IgE triggers local defense reactions. Thus, each of theseisotypes occupies a particular site in the body and has a particular role indefending the body against extracellular pathogens and their toxic products.Antibodies can accomplish this by direct interactions with pathogens or theirproducts, for example by binding to active sites of toxins and neutralizing themor by blocking their ability to bind to host cells through specific receptors.When antibodies of the appropriate isotype bind to antigens, they can activatethe classical pathway of complement, which leads to the elimination of thepathogen by the various mechanisms described in Chapter 2. Soluble immune complexes of antigen andantibody also fix complement and are cleared from the circulation via complementreceptors on red blood cells.