Chapter 2 Antigens Definition of antigen nonself substances

Chapter 2 Antigens

Definition of antigen • “non-self” substances which can combine with antibodies or B-cell receptors or T-cell receptors specifically and have the potential of inducing immune response. • Bacteria, viruses

Immunogenicity Vs. Antigenicity T Ag T Immunogenicity B Plasma cells Sensitized T cells

Hapten and carrier • Low Mw molecules. • Not immuno’genic, unless conjugated to high Mw compounds (carriers) to gain immunogenicity

complete antigen = hapten+carrier

Factors affects immunogenicity

Foreignness Response to Immunization Bovine serum Al’bumin (BSA) Cow NO Sheep Yes ++ Chicken Exceptions: 1. collagen and cytochrome c 2. ‘corneal tissue and sperm Yes +++++

Molecular size • active immunogens : >100, 000 Daltons • poor immunogens: <10, 000 Daltons Exceptions: 1. Gelatin— linear chain, easy to be degraded 2. insulin— 9 ‘aromatic a’mino acid

chemical composition and physical nature • chemical composition – Proteins > polysaccharides > nucleic acids > lipids • Physical nature – Polymer > monomer – Particulates > soluble molecules

Conformation and accessibility

Genotype of the recipient animal • Genes that encode – MHC molecule; – TCR and BCR; – various proteins mechanisms. involved in immune regulatory • That is reason why the different individual has different susceptibility to same disease.

Route of administration • Intracutaneous>subcutaneous>intravenous>oral • skin test—penicillin-penicillium thi’azoline acid

Immunogen dosage • Too little and too much dosage can both induce tolerance. Moderate dosage is the best. • Sometimes repeated administrations are needed— —BOOSTER – Hepa’titis B Virus vaccine: within 24 h; 1 month; 6 month; 7 years

Postulated mode of action of some commonly used adjuvants

Specificity of antigen

Specificity of antigen

Reactivity of antisera with various haptens

Epitopes: antigen regions that interact with antibodies

Epitopes are often small regions of an antigen

Conformational and linear epitopes

B cell and T cell recognize antigens through different mechanisms

Comparison of antigen recognition by T cells and B cells

Common antigen and cross reaction ++++ ++ _

Classification of Ag • According to whether need the help of T cells when B cells produce Ab: – TD-Ag (thymus dependent Ag ) – TI-Ag (thymus independent Ag)

TD-Ag Vs. TI-Ag characteristic TD-Ag TI-Ag Need of T cells and macrophages yes no Chemical nature of antigens most are proteins most are polysaccharides Epitope properties more kinds of epitopes, each number is less composed of T and B cell epitopes composed of repeated B cell epitope Stimulate B cell to produce Ig. G, Ig. M, Ig. A only Ig. M Induce cellular immunity yes no Immune memory yes no TD-Ag TI-Ag

• According to relationship with host: – Xenoantigen – Alloantigen – Autoantigen – Heterophile Ag (Forssman Ag)

Xenoantigen Alloantigen

Forssman Ag guinea pig sheep

Comparison of conventional antigen and superantigen Conventional Antigen Superantigen(SAg) Monoclonal/Oligoclonal response Polyclonal response 1: 104 - 1: 105 1: 4 - 1: 10

Superantigen-mediated crosslinkage of T -cell receptor and MHC II molecules • A SAg binds to all TCRs bearing a particular Vβ sequence regardless of theirantigenic specificity.

Summary 1. Ag & its properties 2. Factors of influencing immunogenicity 3. Specificity and cross reaction of antigen. – antigenic determinate / epitope 4. Classification of antigen : – TI- Ag , TD-Ag, – Heterogenous Ag, alloantigen, autoantigen, SAg 5. Adjuvant