Overview of Antibodies (Immunoglobulins)
- Immunoglobulins are composed of both light and heavy chains arranged into several domains.
- Antigens are recognized by the variable domains of both the heavy and light chains (VH and VL domains) in the Fab portion.
- Within these domains, some regions of both light and heavy chains show exceptional variability in their amino acid composition, which is responsible for the exquisite specificity of antibody to bind specifically with antigen; these regions are called HVRs or CDRs (Figure 1).
- Shown in Figure 2 is a ribbon model of the 3-D structure of the VL, VH, CL, and CH1, domains found in the Fab region of an immunoglobulin.
- Each domain in an antibody molecule has a similar structure of two beta sheets packed tightly against each other and folded in a compressed antiparallel beta structure called an immunoglobulin fold (Figure 2A).
- This fold consists of two antiparallel beta pleated sheets held together by the intrachain S-S.
- It is analogous to a sandwich held together with a toothpick.
- The heavy and light chains associate through interactions between the Ig domains.
- The framework regions form the beta-pleated sheets, and the HVRs in the VH and VL regions form some of the loops that link the beta strands and are clustered at one end of the Fab region (Figure 2B).
- Domains with a very similar structure are present in many other proteins of the body and together constitute the immunoglobulin superfamily of molecules.
Biological Functions of Immunoglobulins
- Immunoglobulins can either be found as transmembrane proteins on the surface of the B cell or they can be secreted by the terminal cell of B cell differentiation, i.e., the plasma cell.
- Immunoglobulins function as antibodies and have the property to combine with the antigen (i.e., immunogen) that triggered their production.
- This unique property of recognition, referred to as specificity, is controlled by an amazing assortment of genes that regulate the production of individual parts of the immunoglobulin molecule by determining the primary amino acid sequence of these components.
- Immunoglobulins are specialized molecules that basically provide two functions:
- (1) an antigen-recognition function, which is carried out by one end of the molecule that binds to antigen, i.e., the two Fab’s
- (2) and an effector function, which is performed by the other end of the molecule by interaction with phagocytic cells, other effector cells and mediator molecules (for example, complement), i.e., the Fc end
- Both these functions are extremely important during the immune response
- The antigen-recognition property of the immunoglobulin molecule confers its exquisite specificity to react with different molecular structures: epitopes which are either linear or more often of a conformational configuration, i.e., antigen
- For example, antibodies directed against the influenza virus recognize neuraminidase (N) and hemagglutinin (H), viral components whose neutralization prevents the virus from adhering to respiratory epithelial cells, thereby destroying the infectivity of the virus.
- This is achieved by producing two separate sets of antibodies, one directed at the N and the other at the H, with different amino acid sequences, each recognizing one of the two unrelated target antigens.
- Antigen recognition by the B cell occurs both at its cell surface through the BCR and by its secreted immunoglobulin product; in contrast, antigen recognition by the T cell occurs only through its surface receptor, i.e., the TCR
- During the course of an immune response to an immunogen specific classes of antibodies are generated at temporally different time periods
- For example, IgM antibody is synthesized early and IgG later by a process referred to as immunoglobulin class switching or isotype switching.
- The genetic mechanism by which this class switching occurs is referred to as somatic recombination or V(D)J recombination
- The effector function of an immunoglobulin is related to the specific isotype produced.
- For example, IgM antibodies synthesized early in the immune response because of their large molecular size are found and function best within the vascular system.
- Other immunoglobulins of lower molecular weight, e.g., the IgG antibodies, produced later in the immune response can readily diffuse between the intravascular and interstitial tissues where they function most effectively.
- Still other antibody molecules, i.e., the secretory IgA, are found at mucosal surfaces where they are produced and function as first lines of defense against pathogens that enter through the mucosal route, as exemplified by influenza, rhinoviruses, and HIV.
The Family of Immunoglobulins Includes Five Functional Classes of Antibodies
- The five different isotypes constitute a family of immunoglobulins, each with a different structure and a different function.
- The individual classes also referred to as isotypes are designated IgG, IgA, IgM, IgD, and IgE.
IgM Is Produced Rapidly Early in the Humoral Response
- IgM is the largest of the immunoglobulin molecules, present in the serum as a pentamer with ten antigen-binding sites and, because of its large size, is restricted almost entirely to the intravascular space.
- These macromolecules are highly efficient agglutinators of particulate antigens, e.g., bacteria and red blood cells, and they activate complement through the classical pathway with a high degree of efficiency.
- This class of immunoglobulin appears to be of greatest importance early in the primary immune response.
- When a foreign antigen is introduced into a host for the first time, the rapid synthesis of IgM antibodies ensures protection before IgG are produced.
- The transition from the production of IgM to IgG and the other isotypes occurs through a ‘‘class switch mechanism’’ involving the interaction of a cascading set of hypermutational events
- The duration of IgM synthesis peaks within a few days and the level of specific serum IgM declines more rapidly than the level of IgG antibodies.
- In one of the primary immune deficiencies, hyperimmunoglobulinemia M syndrome (HIGM), the congenital absence of the CD40 ligand on T cells or of the CD40 molecule on B cells results in the failure of the switch from IgM to IgG, causing the hyperproduction of IgM antibody with diminished production of IgG and the other isotypes, resulting in susceptibility to recurrent bacterial infection
- See Immunopaedia Case Study: An 8 year old boy with recurrent respiratory infections
- See our Immunopaedia discussion and unique graphics describing possible mechansims behind the Hyper-IgM Syndrome
- A maturational delay in the development of the CD40 receptor in the immature fetus and infant is thought to account for the prominent IgM production characteristic of the fetal and newborn immune responses (see Chapter 2).
IgG Is the Predominant Serum Antibody
- The IgG are the most abundant of the immunoglobulins and achieve significant concentrations in both the vascular and extravascular spaces.
- They have a relatively long half-life (t1/2) of ~23 days, cross the placenta, and are able to activate complement through the classical pathway
- This class of immunoglobulin, through its antigen-recognition function, is thought to contribute to protective immunity against many infectious agents, including bacteria, viruses, parasites, and some fungi.
- In addition to its role in the blood, IgG also provides antibody activity in tissues by exerting its effector function.
- Table 1 shows the different IgG isotypes, the concentrations of each and their functional properties.
Table 1. Properties of Human IgG Subclasses
|Normal serum concentration (mg/dL)||540||210||58||60|
|Serum half-life (t½ ) days||21||20||7||21|
|Fc binding capacity on phagocytes||+||-||+||+/-|
|Activation of classical complement pathway||++||+||+++||-|
|Capacity to cross placenta||+++||+||++||+/-|
|Antibody activity||Protein antigens (e.g., diphtheria and tetanus)||Polysaccharide antigens (e.g., pneumococcal and H. influenzae )||Protein antigens (e.g., diphtheria and tetanus)||Polysaccharide antigens (e.g., pneumococcal and H. influenzae )|
- In the human, receptors for the Fc region exist on several types of phagocytic cells, including monocytes, macrophages, dendritic cells, neutrophils, and some lymphocyte subsets, such as NK cells (i.e., lymphocytes that carry out ADCC) (Table 2).
- Target cells coated with IgG antibodies directed against cell surface antigens may be eliminated through this ADCC mechanism.
- This occurs through the action of cytotoxic NK cells, which bind to the surface-coated IgG antibodies through their Fc receptor, thus allowing these cells to come into close contact with and kill the target cells by apoptosis.
Table 2. Summary of the different classes of Fc receptors for each of the major immunoglobulin isotypes together with their molecular and biological properties.
|Receptor name||Principal antibody ligand||Affinity for ligand||Cell distribution||Effect(s) following binding to antibody|
|FcγRI (CD64)||IgG1 and IgG3||High (Kd ~ 10 −9 M)||Macrophages||Phagocytosis|
|Eosinophils||Activation of respiratory burst|
|Dendritic cells||Induction of microbe killing|
|FcγRIIA (CD32)||IgG||Low (Kd > 10 −7 M)||Macrophages||Phagocytosis|
|FcγRIIB1 (CD32)||IgG||Low (Kd > 10 −7 M)||B Cells||No phagocytosis|
|Mast cells||Inhibition of cell activity|
|FcγRIIB2 (CD32)||IgG||Low (Kd > 10 −7 M)||Macrophages||Phagocytosis|
|Neutrophils||Inhibition of cell activity|
|FcγRIIIA (CD16a)||IgG||Low (Kd > 10 −6 M)||NK cells||Induction of antibody-dependent cell-mediated cytotoxicity (ADCC)|
|Macrophages (certain tissues)||Induction of cytokine release by macrophages|
|FcγRIIIB (CD16b)||IgG||Low (Kd > 10 −6 M)||Eosinophils||Induction of microbe killing|
|Follicular dendritic cells|
|FcεRI||IgE||High (Kd ~ 10 −10 M)||Mast cells||Degranulation|
|FcεRII (CD23)||IgE||Low (Kd > 10 −7 M)||B cells||Functions as a possible adhesion molecule|
|(low affinity)||T cells (on activated cells)|
|FcαRI (CD89)||IgA||Low (Kd > 10 −6 M)||Monocytes||Phagocytosis|
|Macrophages||Induction of microbe killing|
|Fcα/μR||IgA and IgM||High for IgM, Mid for IgA||B cells||Endocytosis|
|Mesangial cells||Induction of microbe killing|
|FcRn||IgG||Monocytes||Transfers IgG from a mother to fetus through the placenta|
|Macrophages||Transfers IgG from a mother to infant in milk|
|Dendritic cells||Protects IgG from degradation|