Antigens and immunogens

antigens

bind to components of the immune response but do not elicit an immune response

immunogens

evoke and immune response and bind to components of the innume system

antigenicity

the ability to combine specifically with the products of the humoral and/or cell mediated responses

haptens

small molecules that display antigenicity but not immunogenicity

antigenic determinant or epitope

antigenic site which binds to an antibody or gives rise to the MHC-binding peptide recognized by a T-cell respector

complementary determining region (CDR)

most of the sequence differences in antibodies are confines to three short stretches in the variable (V) region of the heavy and light chains

paratope

the antibody combining site on the antibody that specifically binds to an antigen's epitope.

properties of B cell epitopes

determined by the nature of the antigen binding site on the antibody malecule displayed
can contain sequential or non-essential (conformational) epitopes
nature of antigens are proteins, polysaccarides or lipids

properties of T cell epitopes

do not recognize soluble antigens
antigen must be processed by antigen presenting cells
antigenic peptides recognized by T cells form trimolecular complexes with a T cell receptor and an MHC molecule presenting a peptide
linear or continuous
often internal

characteristics of immunogen

“foreign-ness”
high molecular weight
chemical composition and complexity
degradability

carrier molecule

binds to a nonimmunogenic hapten and makes the hapten immunogenic

subcutaneous

producer of stringest immune response
immunogen processing by Langerhans cellsi

intravenous

immunogen carried to the spleen - either induces immune unresponsiveness or tolerance or immune response

gastrointestinal

produces systemic unresponsiveness

primary response

naive B cells are stimulated by immunogen and differentiate into Ab-secreting cells that produce ABS specific for eliciting Ag. Long lived memory cells are also produced

secondary response

when the same immunogen is encountered by memory B cells, rapid proliferation and differentiation occurs and production of greater quantities of specific Abs occurs than does in the primary immune response

A small Kd

indicates high affinity between Ag and Ab because low concentrations of Ag are required to bind the Ab site

Affinity

the interaction between a single epitope and the combining site on an antibody

avidity

the relative tendency for multiple antibodies and multivalent Ags to combine
cumulative binding strength of all antibody - epitope pairs 

heterophile antibody

an antibody elicited by one antigen can cross-react with a second unrelated antigen

adjuvants

agents that potentiate immune responses
not immunogens
increase the biological half life of vaccine (forms complex and allows for slow release of immunogens)
induce production of loacal inflammatory responses (recruits phagocytic cells)
improve antigen delivery and processing by APC