antigens & antibodies I

antigen

a substance or molecule that can bind to an antibody or T cell receptor

antigenic determinant

the sequence within an antigen that is recognized by lymphocytes

antibody

a molecule that can bind to a specific antigen or immunogen

carrier

a large molecule, usually a protein, to which a small molecule (too small to activate lymphocytes) binds

the small molecule becomes an epitope on the carrier molecule

hapten

small molecules that must bind a protein (“carrier“) to elicit an immune response

immunogen

a type of antigen that can elicit a B cell or T cell immune response

foreignness, high molecular weight, complex

3 properties of substances that determines their immunogenicity

complex

why are proteins are more immunogenic than polysaccharides

T dependent antigens

B cell activation occurs only in the presence of T cell cytokines, and cognate interaction with activated T cells

usually protein

IgM (IgA, IgG, IgE)

the predominant antibody isotype generated when B cells are activated by T dependent antigens

T independent antigens

can activate B cells in the absence of T cell help

usually polysaccharide molecules

(low affinity) IgM

the predominant antibody isotype generated when B cells are activated by T independent antigens

5 kD

the average size of molecules (haptens) that require binding to a carrier (protein) to generate an immune response

penicillin degradation products

clinically significant hapten

can form covalent bonds with protein (carrier); immune response can be generated following recognition of the newly added epitope

• clinical manifestation depends on antibody isotype formed

allergy

the clinical manifestation when IgE is produced following B cell activation to a penicillin proteolytic degradation product

anemia

the clinical manifestation when IgG is produced following B cell activation to a penicillin proteolytic degradation product

epitope

part of an antigen/immunogen that is recognized by adaptive cells in the immune system

also called antigenic determinant

epitope mapping

the identification & characterization of epitopes on the surface of proteins that are recognized primarily by antibodies

linear epitopes

consist of a string of 6 - 12 adjacent amino acids

conformational epitopes

the results of lops and folds formed by the interaction of side chain residues

the epitope is no longer an epitope

it wouldn’t bind as the conformation in which the antigen-binding site the antibody recognizes has been denatured

explain the effect on antibody binding to a conformational epitope when proteins are denatured in laboratory

epitopes must be linear to fit in MHC groove

a conformational epitope it too big to fit into the the MHC groove

T cells recognize peptides in the groove of MHC molecules. explain why the antigen epitopes must be linear and not conformational

B cells epitopes (BCR)

receptor: membrane IgG

MHC required: no

binds soluble antigen: yes

chemical nature of antigens: protein, polysaccharides, lipids, nucleic acids

epitope properties: accessible, hydrophilic, either linear or conformational

T cell epitopes (TCR)

receptor: MHC

MHC required: yes

binds soluble antigen: no

chemical nature of antigens: proteins

epitope properties: linear peptides (short)

mitogen

trigger polyclonal lymphocyte proliferation (produce large numbers of T cells or B cells)

lectins

proteins that bind carbohydrate (sugar) molecules

concanavalin A (Con A) + phytohemagglutinin (PHA)

lectin that induces T cell clonal expansion

pokeweed mitogen (PWM)

lectin that induces T & B cells clonal expansion