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Immunology > Antigens > Flashcards

Antigens Flashcards

1
Q

What parts of pathogens does the adaptive immune system recognize?

A

Antigens.

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2
Q

What is an antigen?

A

Any molecule that can interact with the immunoglobulin receptor of B-cells (or T-cell receptors complexed with MHC)

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3
Q

What is an immunogen?

A

A molecule that induces a specific immune response.

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4
Q

Are all immunogens antigens? Are all antigens immunogens?

A

Yes, no.

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5
Q

Are immunogens and antigens universal, or are they context dependent?

A

Context dependent. Under different conditions, proteins can become antigens or immunogens.

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6
Q

What is the order of immunogenicity for humoral immmunogens (B-cells)?

A

Proteins&raquo_space; Polysaccharides&raquo_space; Lipids & Nucleic Acids

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7
Q

What is the order of immunogenicity for cell mediated immunogens (T-cells)?

A

Proteins (processed peptides in association with MHC molecules).

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8
Q

What are the four properties of all immunogens?

A

Foreignness, molecular size, chemical heterogeneity, degradability.

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9
Q

How does foreignness impact the strength of immunogens?

A

The degree of immunogenicity is dependent upon the degree of foreignness. The greater the phylogenetic distance between species, the greater the chance of immunogenicity (typically). Highly conserved regions may not be immunogenic, and some self molecules may raise an immune response in the animal they came from (sperm, lens tissue).

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10
Q

How does molecular size impact the strength of immunogens?

A

Immunogens must be large enough to be processed.

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11
Q

What is the ideal size of immunogens?

A

Approximately 100 000 Daltons.

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12
Q

How does chemical heterogeneity contribute to the strength of immunogens?

A

Proteins with more complexity in primary structure, and those with secondary, tertiary, or quaternary structure show increased immunogenicity.

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13
Q

How does degradability contribute to the strength of immunogens?

A

Proteins must be degraded to be presented by MHC molecules to activate T-cells. Insoluble molecules are more likely to be phagocytosed. Large molecules are processed more and have more epitopes. L-amino acids are better than D-amino acids as they are more easily processed.

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14
Q

What are adjuvants?

A

Adjuvants are substances that, when injected with antigens, enhance the immunogenicity of the antigens.

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15
Q

Are adjuvants specific to certain antigens?

A

No, they can be used with many different antigens.

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16
Q

What are the three ways that adjuvants increase immunogenicity?

A

They stimulate an immune response, they prolong exposure to the antigen, and they release co-stimulatory signals.

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17
Q

How do adjuvants stimulate an immune response?

A

Freund’s complete adjuvant, containing muramyl dipeptides from cell walls of heat killed mycobacteria, stimulates macrophage activity. This results in production of interleukin-1, which activates helper Tcells.

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18
Q

How do adjuvants prolong exposure to the antigen?

A

Alum and Freund’s adjuvant bind and precipitate the antigen to keep it in the system longer, allowing for the slow release of the antigen. Precipitation also increases size of the antigen, facilitating phagocytosis.

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19
Q

How do adjuvants produce a co-stimulatory signal?

A

Freund’s adjuvant and LPS upregulate co-stimulatory signal systems.

20
Q

What are epitopes?

A

Small, discrete sites on macromolecules that are recognized by lymphocytes (either B-cell or T-cell epitopes).

21
Q

How do B-cells bind to antigens?

A

Directly via cell surface immunoglobulins (lock and key fit)

22
Q

Can B-cell epitopes be nonsequential (conformational)?

A

Yes, B-cell epitopes can be sequential or nonsequential.

23
Q

What properties of epitopes are static for antigens in solution?

A

The epitopes must be topographically accessible on the native molecular surface (hydrophilic). The epitopes must also be flexible and mobile for agglutination (often located on bends and loop structure of protein).

24
Q

How big are linear epitope binding sites?

A

Typically 6-7 amino acids or sugars.

25
Q

How big are conformational epitope binding sites?

A

Typically 15-22 amino acids.

26
Q

Do all epitopes induce an immne response?

A

No, pathogens often make decoys that do nothing but are immunodominant to epitopes that are involved in pathogen function.

27
Q

What are conformational epitopes?

A

Any epitopes that are of secondary, tertiary, or quaternary protein structure.

28
Q

Do T-cells recognize soluble native antigens?

A

No.

29
Q

What types of antigens do T-cells recognize?

A

Those that have been processed and whose peptide fragments are presented in association with MHC molecules. (only linear epitopes)

30
Q

How big are T-cell epitopes?

A

Generally oligomeric peptides of 7-20 amino acids in size.

31
Q

How are antigens recognized by T-cells?

A

Antigens are seens as part of a trimolecular complex - T-cell receptor, antigen, and MHC.

32
Q

What properties must all antigens that activate T-cells have?

A

They must be amphipathic with a hydrophobic region that binds to MHC, and a hydrophilic region that binds to TCR

33
Q

What is the agretope?

A

The MHC binding site of the antigen that binds via hydrophobic amino acids.

34
Q

What is the T-cell receptor binding site that binds via hydrophilic amino acids called?

A

The epitope.

35
Q

Can the T-cell epitopes be internal pathogen proteins?

A

Yes. Since the pathogen must first be digested, both accessible and internal proteins may serve as epitopes for T-cells.

36
Q

Present in antigen-antibody interactions, what is reversible binding?

A

The molecules stick and fall off, stick and fall off, repeatedly.

37
Q

What type of bonds are responsible for antigen-antibody interactions?

A

Ionic bonds, hydrogen bonds, van der Waals interactions, hydrophobic bonds. NO COVALENT BONDS.

38
Q

What is affinity a measure of?

A

The sum total of non-covalent interactions between a single antigen binding site of antibodies and a single epitope. This is dictated by Ag/Ab fit.

39
Q

What is the formula for the association constant measuring affinity?

A

Ka = [Ab-Ag]/([Ab][Ag])

The concentration of antibody bound to antigen divided by the concentration of free antibody and free antigen.

40
Q

How is affinity experimentally measured?

A

Radiolabeled antigen is placed in a semi-permeable membrane that is impermeable to antibodies. A known concentration of antibodies are added to one side of the membrane. The concentration of free antigens will equillibrate on both sides of the membrane. The concentration of bound antigens/antibodies can be determined. This allows for the determination of free antibody concentration.

41
Q

What is avidity a measure of?

A

The stength of multiple interactions between multivalent antibody and antigen. This is dictated by affinity and the number of Ab/Ag bindings.

42
Q

How is avidity calculated from affinity?

A

Avidity = Affinity x # of binding sites.

43
Q

What are the two mechanisms of antigen cross-reactivity?

A

Different antigens may share common epitopes, allowing for one antibody to bind both antigens. The epitopes may be different but share common chemical properties. This also causes the antibody to bind two different epitopes.

44
Q

Describe how antigen cross-reactivity resulted in the ABO blood groups.

A

Anti-A antibodies originate from influenza virus, whose epitopes are similar to the A antigen. Anti-B antibodies result from glycoproteins on gram-negative bacteria (such as E. coli) that resemble B antigens.

45
Q

What is antigen precipitation?

A

When Ag-Ab interactions result in the formation of a lattice structure, forming precipitated antigen and antibody.

46
Q

What are the requirements of antigens and antibodies to form a lattice structure?

A

The antigen and antibody must be bivalent at least.

47
Q

When is maximal precipitation achieved?

A

In the Zone of Equivalence, when there is enough antigen and antibody without excess. This does not mean that the antibody and antigen concentrations need to be equivalent.

Immunology (7 decks)

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