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Antigen-sensitive cells, like lymphocytes, have receptors that do not normally bind to
self-antigens.
Greater differences in molecular structure between a foreign antigen and the body's
own antigens lead to stronger immune responses.
The immunogenicity of a molecule also depends on its degree of foreignness.
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.
Foreign particles are composed of a complex
mixture of proteins, glycoproteins, lipids, and
other components.
Multiple simultaneous immune responses then
targe teach of these components.
Large complex molecules (e.g. proteins) have
surface regions that bind to lymphocyte antigen
receptors and trigger immune responses.
Larger molecules have more epitopes (one
epitope per 5kDa of protein), some more
immunogenic than others.
epitopes or antigenic
determinants.
The most immunogenic are acknowledged while
the rest are ignored. These epitopes are
immunodominant.
Greek haptein (to grasp or fasten)
Small molecules (< 1000 Da) cannot be processed and
presented to the immune system and are non-
immunogenic.
Small molecules that can only bind to larger molecules
to form new epitopes on the larger molecule’s
surface are called haptens.
The larger molecule is called a carrier.
Can cause many drug allergies (drug molecules bind to
normal body proteins).
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.
A small molecule that is antigenic but not
immunogenic on its own
Very small (usually < 1 kDa)
Not immunogenic alone; requires carrier
protein to elicit immune response
Yes – can bind specifically to antibodies
Yes – forms a hapten-carrier complex to be
recognized by immune system
Penicillin, urushiol (poison ivy), some drugs
Usually B-cell mediated (humoral) after
conjugation
Important in drug allergies, autoimmune
diseases
A specific part of an antigen that is recognized
by antibodies or T cells
Small portion of a larger antigen (5–20 amino
acids or sugar residues)
Can be immunogenic as part of the whole
antigen
Yes – site that binds specifically to B-cell or T-
cell receptors
No – it is part of the complete antigen
Influenza hemagglutinin epitope, RBD of SARS-
CoV-2 spike protein
Can be B-cell or T-cell mediated depending on
the epitope
Basis for vaccine development and
diagnostic tests
Unrelated molecules have similar epitopes,
causing antibodies for one antigen to react to
the other.
Blood typing (Bacterial cell wall glycoproteins
have carbohydrate side chains similar to
mammalian red blood cell glycoproteins)
FIP (Feline Infectious Peritonitis) and TGE
viruses (Transmissible gastroenteritis)
Brucella abortus and Yersinia enterocolitica
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.
Antisera to bovine serum and sheep and goat
serum albumin.
Useful for determining evolutionary
relationships.
Occurs when two different antigens
share a similar epitope
Same or structurally similar epitope
on different antigens
Immune system mistakes one
antigen for another due to shared
structure
Antibody or T-cell receptor
confusion based on epitope mimicry
Often linked to molecular mimicry,
autoimmunity, and vaccines
Higher specificity; based on
structural mimicry
Occurs when two different epitopes
are recognized by the same
antibody
Different epitopes but antibody
binds both (low specificity or
overlap)
Antibody binds unrelated antigens
due to cross-reactive binding
properties
Cross-reactive antibody binding to
multiple distinct antigens
Can explain false positives in
diagnostic tests or off-target effects
Lower specificity; based on antibody
binding flexibility
Type I cross-reaction — where a
shared or similar epitope in different
species' immunoglobulin light chains
leads to recognition by the same
antibody.
The closer the species is genetically
(e.g., cow vs. bison), the more likely
they share structurally similar epitopes
→ higher cross-reactivity.
Antibodies made against one species
might accidentally detect antigens in
another species, leading to false
positives or misinterpretation.