Ch' 4' Antigens

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Ch. 4. Antigens AbsHumoral Immune
ResponseB cells Ag ? memory B cells
effector B cells ? plasma cells ? Abs
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• Cell-mediated Immune ResponseT cells Ag ?
  memory T cells effector T cells ?
• CTLs (kill) or
• Th cells (secrete cytokines)

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What is an antigen? a substance that can induce
a specific immune response An immunogen
induces a humoral (B-cell) or cell-mediated
(T-cell) response Haptens are too small to
induce an IR unless coupled to a carrier (an
antigen)
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p. 77
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p. 78
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p. 79
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Properties contributing to immunogenicity 1.
Foreignness 2. Molecular size (molecules lt1000
Da are not usually immunogenic) 3. Chemical
complexity Homopolymers are not good
immunogens Proteins are often good
immunogens. All levels of protein structure
contribute to immunogenicity
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4. Ag processing and presentation
Susceptibility to degradation Can this
antigen be presented? Peptides composed of
D-amino acids are resistant to degradation so
they are not immunogenic Large, insoluble
molecules are more susceptible to phagocytosis
(and thus degradation)
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5. Other factors affecting an immune
response MHC haplotype of animal being
immunized Immunogen dosage - very high or very
low - multiple doses Route of immunization (iv,
id, sc, im, ip) - iv goes to spleen -
subcutaneous goes to lymph nodes Use of
adjuvants alum, IFA, CFA - prolongs antigen
persistence - enhances other aspects of
response - but, granuloma formation (macrophages)
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What is an epitope? It is an Ag
determinant. An immune cell has specificity for
a single determinant a large antigen may have
many epitopes. B cell and T cell epitopes have
different properties chemical
type presentation required surface accessible
or not (see table p. 82)
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p. 82
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p. 82
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p. 76 flu virus is on left, Ab on right
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Antigen-binding sites have different
conformations depending on the nature of the
antigen
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B cell epitopes on native proteins can be
sequential or nonsequential (conformational)
- Usually hydrophilic aas - Usually on
protein surface - Accessible to membrane-bound
or free Ab If a protein is degraded, antibody
binding may be affected
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p. 82
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p. 83
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Haptens are used to study antigenicity Small and
well-defined chemically Can be modified with
precision Used to study cross-reactivity
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Cross-reactivity two unrelated molecules share
epitopes two epitopes are similar compound is
modified so that it is different from original
compound but retains some epitopes (e.g.,
tetanus toxoid)
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Some biologically important molecules act
as haptens - Hormones (peptide and steroid) -
Drugs Hapten-carrier conjugates used to
produce hapten-specific antibodies - e.g., home
pregnancy test for HCG Drug allergies may arise
if hapten-like drug complexes with protein
(e.g., penicillin, aspirin)