Antigen Part I

Immunogenicity

The capacity of antigens to generate

an immunological response

Foreignness

Antigens usually contain molecules or

substances that aren't usually found in the host organism.

Antigenic determinants (epitopes)

: interact with

antigen-specific receptors on immune cells

Diversity

: The chemical makeup, origin, and structure of

antigens vary greatly.

Self- awareness:

Differentiate between non-self

antigens obtained from outside invaders and self-

antigens, which are parts of the host organism's own

cells and tissues.

Processing and Presentation of Antigens:

Specialized

antigen-presenting cells (APCs), such as dendritic cells,

macrophages, and B cells, process and present antigens

to immune cells.

Immunological memory :

ability of the immune system

to remember previous interactions with particular

antigens as a result of exposure to those antigens.

microbial antigen

✓ Bacterial

✓ Viral

✓ Fungal

✓ Parasitic

non-microbial antigen

✓ Food

✓ Dust

✓ Pollen grains

✓ Snake/Mosquite bite

bacterial antigens

Most of the antigenicity of gram- negative bacteria

is associate with the lipopolysaccharide.

lipopolysaccharide

consist of an oligosaccharide attached to a lipid

(lipid A) and to a series of repeating trisaccharides-

lipopolysaccharide

determines the antigenicity of the organism.

genus salmonella

250 serovars based on

antigenicity.

Polysaccharide antigens

– O antigens

Polysaccharide antigens

Bind to TLRs and PRRS

Polysaccharide antigens

Example of endotoxins

Capsule / K antigens

protect bacteria against

phagocytosis and intracellular destruction

pili and fimbriae / f or k antigens

Short projections

pili and fimbriae / f or k antigens

Role in bacterial conjugation and movement

flagella - h antigens

Contains flagellin

flagella - h antigens

Used for bacterial movement

Porins

– proteins that form pores on the surface of gram-negative organisms.

Heat shock proteins

– large amount in stressed bacteria. Response to stressful conditions

(temperature, toxins and infections).

Exotoxins

- highly immunogenic proteins and stimulate the production of antibodies called

antitoxins.

nuclei acid

Bacterial — rich in unmethylated CpG dinucleotides serve both as effective antigens for

adaptive and innate immunity.

Capsid ; capsomeres

— consist of multiple — – good

antigen and capable of stimulating antibody

responses.

envelope

— containing lipoproteins and

glycoproteins.

viral antigens

Gene code for new proteins- some are expressed

on the surface of infected cells.

endogenous antigens

Synthesized inside an animal’s own cells can still

bind to antigen receptors and provoke adaptive

immunity. –

Exogenous antigens

– foreign proteins that enter

outside.

non microbial antigens

Food contain many foreign molecules

non microbial antigens

Inhaled dusts can contain antigenic particles such as pollen grains

non microbial antigens

Organ grafts are foreign to animal

cell surface antigens

Outer membrane of every mammalian cell consists of a fluid lipid bilayer with a

complex mixture of protein molecules embedded to it.

cell surface antigens

Glycoprotein known as blood group antigens are found on the surface of RBCs.

cell surface antigens

Nucleated cells such as leukocytes possess hundreds of different protein molecules on

their surface.

cell surface antigens

The cell surface proteins that trigger graft rejection are called histocompatibility

antigens

Autoimmune response

- an animal may mount immune responses against normal body

components.

Autoantigens

- induces autoimmune response.

autoantigens

Hormones such as thyroglobulin

autoantigens

Structural component such as basement membrane

autoantigens

Complex lipid (myelin)

autoantigens

Intracellular components (mitochondrial components, nucleic acid, or nucleoproteins).

autoantigens

Cell surface proteins such as hormone receptors

size , stability , complexity, foreigness

what makes a good antigen

antigenicity

Ability of the molecules to be antigen.

antigenicity

Varies from one molecule to another.

antigenicity

Influenced by factors such as stability, route of administration,

antigens due to rapid degradation.

large ones

Foreign proteins especially — (1000 Da).

Large proteins; antigens of microorganisms

Large proteins; antigens of microorganisms

clostridial toxin, bacterial flagella, virus capsids, protozoan cell membranes.

size others include

Snake venoms, serum proteins, cell surface proteins, milk and food

proteins, hormones, antibody molecules.

good antigen

More complex carbohydrates may be

effective especially if bound to proteins.

good antigen

Mammalian nucleic acids – relative

simplicity and flexibility, vert rapidly

degraded.

good antigen

Proteins- most effective antigens

good antigen

Complex bacterial lipopolysaccharide

good antigen

Complex proteins- many different amino

acids

good antigen

Structural stability

bad antigen

Simple polysaccharides (starch and glycogen)-

easy degradation

bad antigen

Lipids – wide distribution, relative simplicity,

structural instability and rapid metabolism.

bad antigen

Starch and other simple repeating polymers

bad antigen

Highly flexible molecules that have no fixed

shape

antigen driven ; unstable ; rapidly

Immune responses are , foreign

molecules that are unstable and destroyed very rapidly may not persist for a

sufficient time to stimulate an immune response