What is an antibody?Product of adaptive immunityMade
specifically to bind a unique antigenic epitope (also called an
antigenic determinant)Possesses an antigen binding siteA product of
the Plasma cellMembers of the class of proteins called
immunoglobulins
Receptor-Epitope Interactions and AffinityMost biological
systems can be viewed in terms of interactions between receptors
and epitopes (hand and glove).Specificity of the interaction
between the receptor and epitope is determined by the amino acid
sequence of the receptor (tailoring the glove).If the glove is too
big, the glove falls off.If the glove is too small, you cant get
your hand into it.If the glove is missing fingers
Binding of the epitope in the antigen binding siteGOOD
FITantibody combining siteantigen determinant
Basic Immunoglobulin Structure
The Prototype Immunoglobulin MoleculeHeavy Chains (five types:
Light Chains (two types: andFab fragmentFc fragmentConstant and
variable regionsCarbohydrateDisulfide
linkagesFabFcVHCH1CH2CH3VLCL
Immunoglobulin G (IgG)2 Heavy and 2 Light chains2 Fab and 1 Fc
fragment4 Subclasses (IgG1, IgG2, IgG3, IgG4) Mol. Wt. 150,000
~70-75% of serum immunoglobulin.The major antibody of the secondary
immune responseChange in affinity with time
Immunoglobulin M (IgM)10 Heavy and 10 Light chains10 Fab and 5
Fc fragmentsMol. Wt. ~900,000
Immunoglobulin A (IgA)Monomeric- Serum IgA~15-20% of serum
immunoglobulins2 Heavy and 2 Light 2 Subclasses (IgA1 and
IgA2)Found in serum
Secretory Immunoglobulin A (sIgA)4 heavy chains and 4 light
chains (dimeric)J-Chain and secretory componentThe major
immunoglobulin of secretionsNot found in serumSecretory
ComponentJ-Chain
Immunoglobulin E (IgE)2 Heavy and 2 Light chainsMol. Wt.
~190,000 Trace serum proteinNote CH4 region on H chainAssociated
with atopic or anaphylactic hypersensitivityMay play role in
immunity to helminthic parasitesCH4
Immunoglobulin D (IgD) 2 heavy and 2 light chains
Take Home LessonsWhat is the basic immunoglobulin structure?What
are the characteristics or features of the various fragments of the
immunoglobulin molecule?How is the specificity of an antibody
defined by the amino acid sequence of the immunoglobulin
molecule?What features distinguish each immunoglobulin class?
Study GuideHow are antigenic determinants formed?An immune
response to a single microorganism represents the collection of all
of the immune responses to all of the immunogenic epitopes that
comprise that microorganism.Can you imagine how an immune response
to one microorganism might react with another microorganism?Study
GuideThis is an important concept.Study GuideWhat constitutes a
good fit in an antigen binding site?How does antibody affinity
relate to fit?What are the forces that are attractive and
repulsive?What is actually "fitting" into the combining site?Study
GuideCan you describe the major regions of an immunoglobulin
molecule?What distinguishes the immunoglobulin classes from one
another?What is shared between classes?What is the significance of
the variable regions?What is the significance of the Fab, the Fc,
disulfide linkages?
Study GuideWhat is the function of the J chain?We will talk more
about this immunoglobulin in the Spring term in the Salivary Gland
Course.This immunoglobulin is critical in the protection of mucosal
surfaces.What is unique about the secretory component?Where else
have you seen the J chain? What is its role?We will have a separate
lecture on the role of IgE in atopic hypersensitivity.What is the
importance of the CH4 domain of the heavy chain?
