Question

Include What are the five HCs unique to each class of immunoglobulin-identify the HC and class associated with it. Which is the most abundant and least abundant immunoglobulin? Which has the longest and shorted half life? How many IgG subclasses are there and what structural differences do they posess? What are the major functions of IgG, IgM, gE, IgA, and IgD Which immunoglobulins are monomeric, dimeric and pentamers? Which immunoglobulins cross the placenta? Bind Complement? What is the difference of the two subclasses of IgA? What are their roles? What is secretory lgA and its main functions? Other?

Answer 1

An immunoglobulin test measures the level of certain immunoglobulins, or antibodies, in the blood. Antibodies are proteins made by the immune system to fight antigens, such as bacteria, viruses, and toxins. There are five immunoglobulin classes (isotypes) of antibody molecules found in serum: IgG, IgM, IgA, IgE and IgD.

The body makes different immunoglobulins to combat different antigens. For example, the antibody for chickenpox isn't the same as the antibody for mononucleosis. Sometimes, the body may even mistakenly make antibodies against itself, treating healthy organs and tissues like foreign invaders. This is called an autoimmune disease.

Antibody (or immunoglobulin) molecules are glycoproteins composed of one or more units, each containing four polypeptide chains: two identical heavy chains (H) and two identical light chains (L). The amino terminal ends of the polypeptide chains show considerable variation in amino acid composition and are referred to as the variable (V) regions to distinguish them from the relatively constant (C) regions. Each L chain consists of one variable domain, VL, and one constant domain, CL. The H chains consist of a variable domain, VH, and three constant domains CH1, CH2 and CH3. Each heavy chain has about twice the number of amino acids and molecular weight (~50,000) as each light chain (~25,000), resulting in a total immunoglobulin monomer molecular weight of approximately 150,000.

Immunoglobulin M (IgM), which is found mainly in the blood and lymph fluid, is the first antibody to be made by the body to fight a new infection. Immunoglobulin E (IgE), which is associated mainly with allergic reactions (when the immune system overreacts to environmental antigens such as pollen or pet dander). Heavy and light chains are held together by a combination of non-covalent interactions and covalent interchain disulfide bonds, forming a bilaterally symmetric structure. The V regions of H and L chains comprise the antigen-binding sites of the immunoglobulin (Ig) molecules. Each Ig monomer contains two antigen-binding sites and is said to be bivalent.

The hinge region is the area of the H chains between the first and second C region domains and is held together by disulfide bonds. This flexible hinge (found in IgG, IgA and IgD, but not IgM or IgE) region allows the distance between the two antigen-binding sites to vary.

The five primary classes of immunoglobulins are IgG, IgM, IgA, IgD and IgE. These are distinguished by the type of heavy chain found in the molecule. IgG molecules have heavy chains known as gamma-chains; IgMs have mu-chains; IgAs have alpha-chains; IgEs have epsilon-chains; and IgDs have delta-chains. They are distinguished by the type of heavy chain they contain. IgG molecules possess heavy chains known as γ-chains; IgMs have μ-chains; IgAs have α-chains; IgEs have ε-chains; and IgDs have δ-chains. The variation in heavy chain polypeptides allows each immunoglobulin class to function in a different type of immune response or during a different stage of the body’s defense. The amino acid sequences that confer these functional differences are located mainly within the Fc domain.

Differences in heavy chain polypeptides allow these immunoglobulins to function in different types of immune responses and at particular stages of the immune response. The polypeptide protein sequences responsible for these differences are found primarily in the Fc fragment. While there are five different types of heavy chains, there are only two main types of light chains: kappa (κ) and lambda (λ).

There are five types of mammalian immunoglobulin heavy chain: γ, δ, α, μ and ε. They define classes of immunoglobulins: IgG, IgD, IgA, IgM and IgE, respectively. Heavy chains α and γ have approximately 450 amino acids. Heavy chains μ and ε have approximately 550 amino acids.

Antibody classes also differ in their valency, i.e. the number of arms available to bind antigen. This arises from the ability of certain immunoglobulins to form multimers through linkage of their Fc domains via a J chain. For example, IgM is a pentamer of five identical “Y” shaped monomers. Therefore, the complete IgM protein contains 10 heavy chains, 10 light chains and 10 antigen binding arms (giving IgM a valency of 10).

Antibody classes differ in valency as a result of different numbers of Y-like units (monomers) that join to form the complete protein. For example, in humans, functioning IgM antibodies have five Y-shaped units (pentamer) containing a total of 10 light chains, 10 heavy chains and 10 antigen-binding.

The five subclasses of antibodies are:

1.    Immunoglobulin A (IgA), which is found in high concentrations in the mucous membranes, particularly those lining the respiratory passages and gastrointestinal tract, as well as in saliva and tears.

2.    Immunoglobulin G (IgG), the most abundant type of antibody, is found in all body fluids and protects against bacterial and viral infections.

3.    Immunoglobulin M (IgM), which is found mainly in the blood and lymph fluid, is the first antibody to be made by the body to fight a new infection.

4.    Immunoglobulin E (IgE), which is associated mainly with allergic reactions (when the immune system overreacts to environmental antigens such as pollen or pet dander). It is found in the lungs, skin, and mucous membranes.

5.    Immunoglobulin D (IgD), which exists in small amounts in the blood, is the least understood antibody.

In addition to the major immunoglobulin classes, several Ig subclasses exist in all members of a particular animal species. Antibodies are classified into subclasses based on minor differences in the heavy chain type of each Ig class. In humans there are four subclasses of IgG: IgG1, IgG2, IgG3 and IgG4 (numbered in order of decreasing concentration in serum).

IgA, IgG, and IgM are often measured together. That way, they can give doctors important information about immune system functioning, especially relating to infection or autoimmune disease.

Variance among different subclasses is less than the variance among different classes. For example, IgG1 is more closely related to IgG2, IgG3 and IgG4 than to IgA, IgM, IgD or IgE. Consequently, antibody-binding proteins (e.g., Protein A or Protein G) and most secondary antibodies used in immunodetection methods cross-react with multiple subclasses but usually not multiple classes of Ig.

IgG is the most abundant antibody isotype in the blood (plasma), accounting for 70-75% of human immunoglobulins (antibodies). In humans, there are only two kinds of light chains – κ and λ (based on subtle amino acid differences in the VL and CL regions). The κ and λ chains are found 67% and 33% of the time, respectively. Any antibody can be formed by the association of one heavy chain type with one light chain type. In every possible combination there will be two identical heavy and light chains in the antibody unit (monomer). Hence the IgM pentamer can either comprise (μ2κ2)5 or (μ2λ2)5.

Immunoglobulins are further broken down into four subclasses designated IgG1, IgG2, IgG3 and IgG4 (listed in decreasing order of abundance in the serum). They share more than 95% sequence homology in the CH regions of the γ-heavy chains. There are also two subclasses of IgA: IgA1 (90%) and IgA2 (10%). Serum IgA is a monomer but is found in secretions such as tears, mucous and saliva as a dimer. In secretions, IgA has a J chain and another protein called the secretory piece (or T piece) associated with it. In addition, several subclasses of κ and λ light chains are known to exist.

Once an antibody is produced against a specific antigen, the next time that antigen enters the body, the immune system "remembers" its response and produces more of the same antibodies. In that way, checking for the presence of specific immunoglobulins in the blood can be helpful in diagnosing or ruling out infections or certain other illnesses.

Doctors also rely on the immunoglobulin test as one of the tools to help diagnose immunodeficiencies (when the immune system isn't working properly). A person can be born with an immunodeficiency or acquire it through infection, disease, malnutrition, burns, or as a side effect of medicines. Doctors may suspect an immunodeficiency in a child who experiences frequent or unusual infections.

Immunoglobulin levels are also used as part of an evaluation for autoimmune conditions such as juvenile idiopathic arthritis, lupus, and celiac disease. Immunoglobulins are heterodimeric proteins composed of 2 heavy and 2 light chains. They can be separated functionally into variable domains that bind antigens and constant domains that specify effector functions, such as activation of complement or binding to Fc receptors. The variable domains are created by means of a complex series of gene rearrangement events and can then be subjected to somatic hypermutation after exposure to antigen to allow affinity maturation. Each variable domain can be split into 3 regions of sequence variability termed the complementarity-determining regions (CDRs) and 4 regions of relatively constant sequence termed the framework regions. The 3 CDRs of the heavy chain are paired with the 3 CDRs of the light chain to form the antigen-binding site, as classically defined. The constant domains of the heavy chain can be switched to allow altered effector function while maintaining antigen specificity. There are 5 main classes of heavy chain constant domains. Each class defines the IgM, IgG, IgA, IgD, and IgE isotypes. IgG can be split into 4 subclasses, IgG1, IgG2, IgG3, and IgG4, each with its own biologic properties, and IgA can similarly be split into IgA1 and IgA2.

Placental transfer of maternal IgG antibodies to the fetus is an important mechanism that provides protection to the infant while his/her humoral response is inefficient. IgG is the only antibody class that significantly crosses the human placenta. This crossing is mediated by FcRn expressed on syncytiotrophoblast cells. There is evidence that IgG transfer depends on the following: (i) maternal levels of total IgG and specific antibodies, (ii) gestational age, (iii) placental integrity, (iv) IgG subclass, and (v) nature of antigen, being more intense for thymus-dependent ones. These features represent the basis for maternal immunization strategies aimed at protecting newborns against neonatal and infantile infectious diseases. In some situations, such as mothers with primary immunodeficiencies, exogenous IgG acquired by intravenous immunoglobulin therapy crosses the placenta in similar patterns to endogenous immunoglobulins and may also protect the offspring from infections in early life. Inversely, harmful autoantibodies may cross the placenta and cause transitory autoimmune disease in the neonate.