Humoral Immunity

Antigen

-any molecule that can interact with products of immune response

-interaction between receptor (Ab or TCR) and ligand (antigenic epitope)

B cell epitope (2 types)

-Linear: AA residue is adjacent in polypeptide chain

-discontinuous: AA residue in different parts of polypeptide chain

T-dependent (TD) Ags

response requires T cell help (cytokines), generates immunological memory

T-independent (TI) Ags

-response does not require T cells

-B cell response generally has no memory cells (need cytokines)

-has lipopolysaccharide (LPS)=not AG specific

immunogenic

ability to induce an Ab, or CMI (cell mediated immunity) response on its own

most potent immunogens

-Proteins, are the normal activator (immunogen) of CMI

-polysaccharides are second best

What makes up an antigen (4)

-forgiveness=greater polygenetic differences

-molecular size=the larger the better

-chemical composition(complexity)= homopolymers are NOT GOOD immunogens

susceptibility to AG processing/presentation= D-AA are NOT easily degraded, L-AA are easily degraded

Haptens

-small molecules that are antigenic but NOT immunogenic

-univalent

-often need to be attached to carrier protein to be immunogenic

-ex: penicillin, catechol

Epitopes

• a given Ag can have multiple T and B cell epitopes

• complex Ag=more epitopes

• epitope=the specific part of an Ag the Ab recognizes

ideal antigen properties

generally, the larger and more complex=better

B and T cell antigens are usually:

B cell: protein or carbohydrate

T cell: protein or peptide

Ig vs Ab

Ig= generic term

Ab= specifies an Ig with a particular antigen specificity

Antigen specificity of a B cell results from:

Results from membrane bound Ig

Activated B cell develops into:

-Develops into a plasma cell secreting soluble Abs

-a given B cell makes Abs with ONE antigen specificity (binds one epitope)

Clonal selection and clonal proliferation allow for:

Expansion of B cells specific for one antigen

Idiotype

the area of Ab where Ag binds

Ab structure (chains)

2 identical heavy chains, 2 identical ight chains

Ab sturcutre, 2 heavy + 2 light chains

Ab structure (bonds for heavy chains)

Heavy chains are bonded by disulfide bonds in hinge region

Hinge region where heavy chains are bonded by disulfide bonds

Ab structure (bonds for light chains)

Light chains are bonded by a disulfide bond in CL region

Light chains are bonded by a disulfide bond in CL region

5 classes of heavy chains (inner)

-Isotypes: IgG, IgM, IgA, IgE, IgD (GAMED)

-aka: gamma, nu, alpha, epsilon, delta

2 classes of light chains (outer)

kappa or lambda

Where Ag binds on Ab

-Ab binds Ag at N-terminal end

-Fab region binds Ag

Ag binding to N-terminus occurs in Fab region

Fc biological activity

Has C’ activation and ADCC (antibody dependent cellular cytotoxicity) activity

Fc has biologic activity

Ab-Variable regions

• N-terminal region of Ig: binds to Ag (VH and VL)

• variability is concentrated in 3 complementarity determining regions (CDRs): CDR1, CDR2, CDR3

• Framework regions= area that are less variable

CDRs of VH and VL chains (CDR1,CDR2,CDR3)

Monomer:bivalent Ig isotypes

IgG, IgD, IgE

Dimer/Pentamer/Multivalent Ig Isotypes

IgA(dimer) and IgM (pentamer)

IgM (5)

-pentameric (10 Ag binding sites)

-appears first during primary immune response

-best at C’ (complement) activation

-best at agglutinating (clumping) Ab

-monomer on surface of mature B cell

IgG (5)

• divalent monomer with 2 Ag binding sites

• predominant Ig in blood and lymph

• appears in late primate and early secondary immune response

• opsonin and mediates ADCC, and activates C’

• crosses placenta

IgA (3)

• a secretory Ig, in secretions (ex: oral cavity)

• monomer on B cell surface, dimer in secretions (can be tetramer)

• secretory component (SC): protective protein

IgD (3)

• monomer

• 2nd receptor on B cell (after IgM, co-expressed with IgM)

• clearly defined function is UNKNOWN (likely involved with activation of naive B cells)

IgE (5)

• binds to basophils and mast cells

• LOWEST concentration in serum

• short ½ life in blood, long ½ life in mast cell surface

• igE binds Ag=degranulation of mast cell=ALLERGIC REACTION

• protects against parasitic worms=causes acute inflammatory reaction

Antibodies are produced by:

B cells

Fab region variability:

3 hypervariable regions: CDR1, CDR2, and CDR3

Millions of different antibodies are made by:

Ig gene arrangement

Gene segments encoding the heavy chain

• V (variable)

• D (diversity)

• J (joining)

• C (constant)

• VDJ= variable for heavy chain

Gene segments encoding light chain

• V,J,C

• VJ=variable domain in light chain

Selection of genes is:

random and occurs before Ag stimulation

Alleic exclusion

B cell will only express one allele of the Ig H and L genes

Class switching

after Ag stimulation, a mature B cell can change its isotype

Gene arrangement possibilities

genes alone: 1.5×10^7 possibilities

V(D)J recombinase

a complex of enzymes important in recognizing and excising the intervening sequences

RAG1 and RAG2

• RAG=recombinant activating genes

• expressed in B and T cells

• recognize and bring together VDJ genes

Terminal deoxynucleotidyl transferase (Tdt)

• adds back nucleotides in the joining region

• non-germline nucleotide bases

Generation of Ig diversity (4)

• multiple germ-line gene segments

• junctional flexibility

• N-region nu addition

• P-region nu addition

Multiple germ-line gene segments

VDJ, about 1-3.5×10^7 possibilities

junctional flexibility (variability)

imprecise joining of VDJ or VJ regions, and yield different AA in this region=diversity

N-region addition

addition of random bases by Tdt (during H chain rearrangement)

P-region addition

addition of bases by repair enzymes

Somatic hypermutation

• mutation occurs in mature B cell when it proliferates

• after Ig gene rearrangement, better Ag binding

• occurs during clonal proliferation

mature (naive) B cell expresses:

IgM and IgD on its surface

Upon Ag recognition, B cell will divide and produce:

Will produce plasma cells and secrete Abd and produce memory cells

Class switch

• dependent on presence of T cell cytokines

• upon next Ag exposure, memory B cell can develop into plasma cell secreting another Ig isotype

igM and IgD are first expressed:

In the constant region sequences closest to the VDJ sequences

Ag independent phase

Ig gene arrangement= Ag specific B cells

Ag dependent phase

• movement to periphery (90% of B cells die)

• B cells develop into plasma or memory cells (class switching)

B cells produce:

Antibodies (effector cells)

Primary response

• when naive B cell first sees its Ag

• IgM production

• slower response, 7-14 days

• T cell help NOT required

• B cells often class switch

Secondary response

• memory response

• produce IgG or other isotypes

• affinity maturation

• quicker response, 1-3 days

• NEEDS T cell help

Development of memory cells and isotype class switch is controlled by:

T cells, cytokines, like IL-4

primary vs secondary immune response

Ig effector functions (6)

• pathogen neutralization

• agglutination(cells)/precipitation(soluble antigens)

• complement(C’) fixation

  • inflammation=opsonization=cell lysis

• antibody-dependent cellular toxicity

  • death by NK cells, neutrophils or eosinophils

• opsonization

  • stimulated phagocytosis and agglutination

• inflammation

  • granule release from mast cells—>allergies