Antigen Part II

foreigness

Antigen-sensitive cells, like lymphocytes, have receptors that do not normally bind to

self-antigens.

foreigness

Greater differences in molecular structure between a foreign antigen and the body's

own antigens lead to stronger immune responses.

foreigness

The immunogenicity of a molecule also depends on its degree of foreignness.

foreigness

Kidney grafts from identical twins are readily accepted due to identical proteins, while

grafts from unrelated individuals or different species are rejected unless

immunosuppressive drugs are used.

epitopes

Foreign particles are composed of a complex

mixture of proteins, glycoproteins, lipids, and

other components.

epitopes

Multiple simultaneous immune responses then

targe teach of these components.

epitopes

Large complex molecules (e.g. proteins) have

surface regions that bind to lymphocyte antigen

receptors and trigger immune responses.

epitopes

Larger molecules have more epitopes (one

epitope per 5kDa of protein), some more

immunogenic than others.

antigenic

determinants.

epitopes or

epitopes

The most immunogenic are acknowledged while

the rest are ignored. These epitopes are

immunodominant.

haptens

Greek haptein (to grasp or fasten)

haptens

Small molecules (< 1000 Da) cannot be processed and

presented to the immune system and are non-

immunogenic.

haptens

Small molecules that can only bind to larger molecules

to form new epitopes on the larger molecule’s

surface are called haptens.

haptens

The larger molecule is called a carrier.

haptens

Can cause many drug allergies (drug molecules bind to

normal body proteins).

haptens

Many potential haptens so animals combat this by

making diverse antibodies and antigen receptors.

Penniciloyl groups (from penicillin) bind to serum

proteins like albumin. Can cause penicillin

allergies.

Resin (urushiol) of poison ivy plant binds to skin

proteins of person. Lymphocytes attack this and

cause allergic contact dermatitis.

hapten

A small molecule that is antigenic but not

immunogenic on its own

hapten

Very small (usually < 1 kDa)

hapten

Not immunogenic alone; requires carrier

protein to elicit immune response

hapten

Yes – can bind specifically to antibodies

hapten

Yes – forms a hapten-carrier complex to be

recognized by immune system

hapten

Penicillin, urushiol (poison ivy), some drugs

hapten

Usually B-cell mediated (humoral) after

conjugation

hapten

Important in drug allergies, autoimmune

diseases

epitope

A specific part of an antigen that is recognized

by antibodies or T cells

epitope

Small portion of a larger antigen (5–20 amino

acids or sugar residues)

epitope

Can be immunogenic as part of the whole

antigen

epitope

Yes – site that binds specifically to B-cell or T-

cell receptors

epitope

No – it is part of the complete antigen

epitope

Influenza hemagglutinin epitope, RBD of SARS-

CoV-2 spike protein

epitope

Can be B-cell or T-cell mediated depending on

the epitope

epitope

Basis for vaccine development and

diagnostic tests

type 1 cross reaction

Unrelated molecules have similar epitopes,

causing antibodies for one antigen to react to

the other.

type 1 cross reaction

Blood typing (Bacterial cell wall glycoproteins

have carbohydrate side chains similar to

mammalian red blood cell glycoproteins)

type 1 cross reaction

FIP (Feline Infectious Peritonitis) and TGE

viruses (Transmissible gastroenteritis)

type 1 cross reaction

Brucella abortus and Yersinia enterocolitica

type 2 cross reaction

Related animal species may have protein

epitopes with only minor differences with each

other causing antibodies directed at one species

to react in another related species.

type 2 cross reaction

Antisera to bovine serum and sheep and goat

serum albumin.

type 2 cross reaction

Useful for determining evolutionary

relationships.

type 1 cross reaction

Occurs when two different antigens

share a similar epitope

type 1 cross reaction

Same or structurally similar epitope

on different antigens

type 1 cross reaction

Immune system mistakes one

antigen for another due to shared

structure

type 1 cross reaction

Antibody or T-cell receptor

confusion based on epitope mimicry

type 1 cross reaction

Often linked to molecular mimicry,

autoimmunity, and vaccines

type 1 cross reaction

Higher specificity; based on

structural mimicry

type 2 cross reaction

Occurs when two different epitopes

are recognized by the same

antibody

type 2 cross reaction

Different epitopes but antibody

binds both (low specificity or

overlap)

type 2 cross reaction

Antibody binds unrelated antigens

due to cross-reactive binding

properties

type 2 cross reaction

Cross-reactive antibody binding to

multiple distinct antigens

type 2 cross reaction

Can explain false positives in

diagnostic tests or off-target effects

type 2 cross reaction

Lower specificity; based on antibody

binding flexibility

type 1 cross reaction

— where a

shared or similar epitope in different

species' immunoglobulin light chains

leads to recognition by the same

antibody.

higher cross-reactivity.

The closer the species is genetically

(e.g., cow vs. bison), the more likely

they share structurally similar epitopes

→

false

positives or misinterpretation.

Antibodies made against one species

might accidentally detect antigens in

another species, leading to