Immunology L3, Antibodies

how many constant domains on heavy vs light chains?

3 constant domains on heavy chain, one on light chain

Ig molecules as both antibodies and antigens

Because Ig molecules are glycoproteins, they can be antigens as well as antibodies. For example, human Ig injected into a mouse will be recognized as foreign by the mouse's immune system, and antibodies will be produced against the human Ig. The antibodies produced would be referred to as "mouse anti-human immunoglobulin" (mouse alpha-human Ig). Another example is autoimmune (reacting against self Ag) reactions that result in antibody synthesized against self Ig. This occurs in rheumatoid arthritis and these anti-Ig antibodies are called "rheumatoid factor". Immunoglobulin molecules are, themselves, very good immunogens if placed into a heterogeneous host and can elicit an immune response in certain instances.

how does IgG do opsinization?

By coating an antigen with IgG, it becomes more easily ingested by phagocytic cells such as neutrophils and macrophages. Some subclasses of IgG are better at this function than others. IgG opsinizes/tags the foreign substance for phagocytosis by another cell

agglutination

Clumping of microorganisms or blood cells, typically due to an antigen-antibody interaction; IgM does this best

Ch, Cl, Vh, Vl

Constant regions of the heavy and light chains are abbreviated CH and CL, respectively. Variable regions of heavy and light chains are abbreviated VH and VL, respectively

structure of IgE

Due to the presence of an extra constant heavy chain domain, the lack of a hinge region, and numerous carbohydrate moieties, IgE is similar in gross structure to monomeric IgM. Half-life = 2 days.

functional parts of an Ig molecule

Fab and Fc; Fab = antigen-binding portion; Fc = confers biological activity

Fab vs F(ab)2

Fab binds one Ag; F(ab)2 binds two Ag

heavy chains

Five major heavy chain Ig isotypes or classes of antibodies have been identified. They are, in descending order of concentration in normal human serum, IgG, IgA, IgM, IgD, and IgE (remember "GAMDE"). IgG and IgA may be divided into subclasses (in descending order of serum concentration, IgG1, IgG2, IgA1, IgG3, IgG4, IgA2). IgG is by far the most prominent isotype found in the peripheral circulation. As such, it was the first to be discovered, and its structure was the first to be elucidated.

which Ig is in secretions?

IgA

IgA function

IgA is the predominant antibody found in bodily secretions such as tears, saliva, bronchial secretions, bile and intestinal fluid, colostrum and milk, and in the mucosa of the genital tract. Protects the host from attack of pathogens at mucosal surfaces. The neutralizing properties of IgA are very good, and it is relatively good at agglutinating. Does not fix complement, does not act as an opsonin. NOT efficient at activating complement or opsonization. Half life = 5.5 days

Which isotypes would be affected by a genetic deficiency in J chain?

IgA, IgM

structure of different IgA subtypes

IgA1 = mono or polymeric IgA2 = dimer + SC

subclasses of IgA

IgA1 and IgA2

which IgA combines with SC?

IgA1 does not, IgA2 does

proportion of IgA subclasses in milk

IgA1: 40% IgA2: 60%

proportion of IgA subclasses in serum

IgA1: 90% IgA2: 10%

function of IgD

IgD secretion is not part of the conventional immune response. It is found in plasma in very low levels and is normally found on the surface of B cells in association with IgM where it functions as an antigen receptor. Its presence serves as a marker of B cell differentiation. Half-life = 2.8 days.

which Ig is for allergies?

IgE

most prominent isotype found in the peripheral circulation

IgG

which Ig opsinizes?

IgG

name the different isotypes

IgG IgA IgM IgD IgE

which antibodies can cross the placenta?

IgG (though not really IgG2)

% of total IgG half-life in days

IgG1 = 70 IgG2 = 20 IgG3 = 7 IgG4 = 3

which IgGs are best at opsinization?

IgG1 and IgG3 are best, IgG2 can do some, IgG4 may or may not do it

subclasses of IgG

IgG1, IgG2, IgG3, IgG4

IgG C' binding

IgG3 is the best, IgG1 and IgG2 can do some, IgG4 does not do it

serum half-life in days of different Ig isotypes

IgG: 23 IgA: 5.5 IgM: 5 IgD: 2.8 IgE: 2

which Ig neutralizes?

IgM best, then IgG, then IgA

IgM function

IgM is the first and major Ig class formed in the initial immune response to antigen (primary response). Due to its large size, IgM is excellent at neutralization of toxins, viruses, etc. Highly effective at complement activation by the classical pathway. Does not cross the placenta. Does not normally leave the vascular compartment due to large size. Good at agglutination. Half-life = 5 days

which Ig responds to an antigen first?

IgM predominates in the primary response to antigen

which Ig agglutinates best?

IgM the most, then IgA, then IgG (IgD and IgE do not agglutinate)

parts of an antibody

Immunoglobulin G has gamma heavy chains, IgA has alpha heavy chains, IgM has mu heavy chains, IgD has delta heavy chains, and IgE has epsilon heavy chains. All light chains are either kappa or lambda light chains, either of which may be found on any Ig molecule, regardless of isotype. Each Ig unit is made up of 2 heavy chains, 2 light chains, and has 2 antigen-binding sites.

places you find immunoglobulins

Immunoglobulin molecules can either be secreted by plasma cells or remain anchored in the cell membrane of the B cell. In the latter case, the Ig molecule will act as an antigen receptor on the surface of the B cell.

J chain

The joining chains of the IgM and IgA antibodies

IgG function

The major Ig found in serum and extravascular fluids; neutralization of toxins, viruses, bacterial attachment factors, etc.; opsonization; complement activation via the classical complement activation pathway; crosses placenta, therefore is important in immunity of the fetus and neonate; can penetrate into extravascular spaces; Ab-dependent cell mediated cytotoxicity (ADCC); half-life = 23 days

light chains

There are two types of light chains: Kappa (κ) or Lambda (λ). Any light chain can pair with any heavy chain.

The idiotype of an antibody molecule is determined by the amino acid sequence of the:

Variable region of the heavy and light chains

IgM structure

When secreted by plasma cells, IgM is a polymer formed by five antibody subunits linked by disulfide bonds to form a pentamer. A small peptide called the J chain joins the units together. Also found as a monomer on the surface of B cells where it functions as an antigen receptor. Has no hinge region, but has an extra constant domain in the heavy chain. Because IgM lacks a hinge region, it is not flexible and can effectively bind 5 antigens at a time.

structure of IgD

With the exception of having one less disulfide bond and numerous carbohydrate moieties, IgD is similar in gross structure to IgG. The only Ig susceptible to proteolytic degradation. 2.8 day serum half-life

IgG structure

Y-shaped molecule composed of four polypeptide chains held together by disulfide bonds; two of the polypeptide chains (heavy chains) are roughly twice the size of the other two (light chains)

Ig shape

Y-shaped molecule composed of four polypeptide chains that are held together by disulfide bonds; two of the polypeptides are larger and are called "heavy chains" and two polypeptides are smaller and called "light chains"; both heavy chains are always identical, and both light chains are always identical

idiotype

a unique antigen binding pocket/shape created by the heavy chain and light chain; determined by the regions of an Ig molecule that come in contact with an epitope

bad outcomes of antibody production

allergies immune complex disease autoimmunity

molecular shuffling and recombination

allows diverse antibodies to be made while still conserving space in the genome; involves the splicing and annealing of relatively few genes; mediated by V(D)J recombinase

what different types of antibodies can one B cell make?

always the same idiotype (specific for a particular epitope) but can change the isotype (Fc region)

antibody vs immunoglobulin

antibodies are immunoglobulins made in response to a stimulus; the broader class of proteins are immunoglobulins and antibodies are a subset of immunoglobulins; terms are used interchangeably

where do you find IgD?

cell-bound antigen receptor on B cells

how does pepsin cleave Ig?

cleaves below the disulfide bonds to make a F(ab)2 which can bind two identical antigens

biological activity of antibodies

conferred by Fc portion; can include the ability to help phagocytes "eat" microorganisms, help lyse organisms, and even help clump organisms together

Ch region

constant heavy chain region; "downstream" of the VDJ region of the immunolglobin locus; encode the heavy chains that determine isotype; there are 9 CH genes that correspond to the different immunoglobulin isotypes. The mu and delta genes are the ones closest to the V region genes and are the ones transcribed first during B cell development

F(ab)2

created when the Ab molecule is cleaved by pepsin below the disulfide bonds and can bind two identical antigens

variable region of Ig

found in the amino terminus of each heavy and light chain molecule (in the Fab fragment) where the amino acid sequences are highly variable; Ig binds to antigen here; high degree of sequence variation in the Fab region is one factor that allows one to mount antibody responses to a myriad of antigens (i.e. responsible for antibody diversity)

where are B cells made?

from 6 months gestation through rest of life, B cells are made in the bone marrow

what are antibodies?

glycoproteins made by B cells; synthesized in response to a foreign antigen (Ag); able to specifically "recognize" and bind to the Ag which induced its production

light chain rearrangement

happens after heavy chain rearrangement; light chain variable region only consists of VL and JL regions; VDJ recombinase recombines gene segments encoding the VL and JL regions to generate a unique light chain within a B cell; there are 40 variable (V-kappa) genes and 5 joining (J-kappa) genes for 200 different combos.; there are 40 variable (V-lambda) and 4 joining (J-lamba) genes for 160 different combos; there are a total of 360 unique recombination possibilities for light chains, which contributes to the diversity of the antibody molecules produced

heavy chain rearrangement

happens before light chain rearrangement; heavy chain variable region requires three gene segments to be rearranged: Variable (V), Diversity (D), and Joining (J); VDJ recombinase cuts and pastes these 3 segments into a contiguous exon that can be transcribed and translated to a unique heavy chain; the human heavy chain loci consists of 50 VH, 20 DH and 6 JH gene segments; any V segment can be combined with any D segment which can also be combined with any J segment for a total of 6,000 unique recombination possibilities (50 VH X 20 DH X 6 JH = 6,000), which contributes to the diversity of the variable region of the heavy chain

constant region of Ig

has an amino acid sequence that is highly consistent within a species; heavy chain is made up of multiple domains with highly conserved AA sequence; light chain is made up of one domain with highly conserved AA

three main regions of an Ig molecule

hinge region, constant region, variable region

where is IgA predominantly found?

in bodily secretions: tears, saliva, bronchial secretions, bile and intestinal fluid, colostrum and milk, and in the mucosa of the genital tract

B cell development

in the bone marrow; undergo rearrangements of both the heavy chain and light chain variable regions; heavy chain is always rearranged first, followed by the light chain; VDJ recombinase catalyzes the "cutting and pasting" of the regions that encode the heavy chain and light chain to generate unique combinations of variable regions for the developing B cell repertoire

hypervariable region

in the variable region of Ig; region of extreme amino acid sequence variability; contact with the epitope of an antigen occurs here; Ag-Ab interactions are non-covalent

breastfeeding and IgA

infants who are breastfed are much less likely to contract certain types of infections due to the presence of IgA and other factors in breast milk

isotype vs idiotype

isotype: classes of antibodies based on only the type of heavy chain; IgG, IgA, IgM, IgD, anf IgE; due to Fc idiotype: unique antigen binding pocket/shape created by the heavy chain AND light chain; due to Fab

V(D)J recombinase

mediates molecular shuffling and recombination; a set of enzymes; two of the component proteins are produced by RAG1 and RAG-2 (recombination-activating genes)

which isotypes have the highest concentration in normal human serum?

most IgG IgA IgM IgD IgE least (remember "GAMDE").

which Ig activates compliment?

mostly IgM, a little IgG

good outcomes of antibody production

neutralize viruses accelerate clearance of bacteria neutralize toxins used as therapies to kill tumor cells used for clinical assays

IgA structure

normally found as a dimer of two subunits linked by a J chain which is produced by the B cell; in bodily secretions, IgA usually also has a secretory component that is produced by epithelial cells; the secretory component is a glycoprotein that protects IgA from proteases found on body surfaces and intestinal fluid

what makes antibodies?

only B cells of the immune system (type of lymphocyte); each B cell produces antibody specific only for a given epitope and no other; it may produce epitope-specific antibody of IgM (first) and then IgG or IgA or IgE, etc.: the isotype produced by a single B cell may change but never the idiotype

which Ig crosses the placenta?

only IgG; IgG1, IgG3, and IgG4 all cross; IgG2 may or may not cross

Fab

part of the Ig; antigen-binding part; determines the idiotype (unique antigen binding pocket/shape created by the heavy chain and light chain); can bind to one antigen; a F(ab)2 is created when the Ab molecule is cleaved by pepsin below the disulfide bonds and can bind two identical antigens; specificity for Ag of the Fab is the same (since the two light chains vs two heavy chains are identical)

Fc

part of the Ig; crystallizable fragment; contains only the constant region; found at the carboxy terminal; within Ch2, contains complement binding site; determines isotype; contains biological activity of the Ab molecule (e.g. complement binding, crossing the placenta, and binding to cells)

pepsin vs papain

pepsin cuts below the disulfide bond, heavy chains are kept together, get F(ab)2 which can bind two identical antigens; if you instead cut with papain, cuts above disulfide bond, get two Fab fragments, can bind antigens independently

function of IgE

present in very low concentrations in normal human serum; mediates allergic reactions (Type 1 hypersensitivity) by binding certain antigens (termed allergens); initiates release of histamines and other inflammatory agents from basophils and mast cells; IgE and eosinophils are important in defense against parasitic infections; Play an important role in Type 1 hypersensitivity responses (allergies) via the extra CH domain binding with very high affinity to Fce receptors found on mast cells and basophils. Does not agglutinate or activate complement.

functions of antibodies

resolve infection

what determines the type of heavy chain?

the amino acid sequence

what determines the isotype?

the type of heavy chain in the Fc region