Antigen Structure and Antibody Interactions Flashcards
Immunogen
Generates either a humeral or cellular immune response
Antigen
Reacts with antigen receptors, irrespective of its ability to generate immune response: may or may not be immunogenic
Hapten
Small molecule able to react with preformed antibodies
Not immunogenic by itself, must be complexed to large protein (carrier)
Immunogenicity of antigens
Proteins > polysaccharides (B cells and Ab responses)
Proteins: T cell responses
Lipids and nucleic acids only immunogenic when complexed with proteins or polysaccharides
Properties of an antigen
Foreignness
Molecular size (>100kDa)
Chemical complexity and composition
Susceptibility to processing and presentation
Biological factors affecting immunogenicity
Genotype of recipient
Immunogen dosage
Rout of administration
Adjuvants
Adjuvants
Substances that when mixed with and injected with an antigen enhance the immunogenicity of that antigen
- Prolonged antigen persistence
- Enhance costimulatory signal
- Increase local inflammation, resulting in macrophage activation and antigen presentation
- Stimulate nonspecific proliferation of lymphocytes
Epitopes
Antigen receptors on lymphocytes recognize epitopes (antigenic determinants)
Usually immunologically active regions of an immunogen
Antigen recognition by B and T cells are fundamentally different and does not involve the same epitopes
B Cell Epitopes
Accessible and hydrophilic epitopes
Linear sequences of aa or non-sequential or conformation epitopes
Weak, non covalent interactions due to complementary shape between epitope and antigen-binding site
Can depend on shape of globular protein
Found on flexible region of immunogen
May have many different epitopes
T Cell Epitopes
Peptides complex with self MHC molecules on the surface of antigen presenting cells and other nucleated cells
Often internal and contain amphipathic peptide sequences revealed during processing
Hydrophobic residues interact with MHC molecules while hydrophilic regions interact with T cell receptor
Immunodominance is determined by ability to interact with MHC molecules of a given individual
Some T cells can recognize lipids and glycolipids presented by CD1
Mitogens
Capable of activating many clones of B cells or T cells, irrespective of their antigenic specificity and are known as polyclonal activators
Some prefer T or B cells
Superantigens
Activate large numbers of T helper cells by cross-linking their T cell receptors with any MHC2 molecule on an antigen presenting cells
Antigen-antibody interactions
Mediated by weak forces determined by close approach and multiple interactions
Exclusion of water molecules at site of interaction further increases the magnitude of weak atomic forces
NO COVALENT BONDS
Avidity
Overall strength of binding between a multivalent antibody and multivalent antigen
Cross-reactivity
antibodies in a polyclonal antibody preparation raised against an antigen can cross-react with a partial related antigen than bears one or more identical/similar epitopes