Chapter 3: Antigens and MHC Flashcards
What is an antigen
a foreign substance recognized by the immune system
what is an immunogen
a substance capable of causing an adaptive immune response
what is an epitope
a marker on an antigen that triggers an antibody response
what is the major histocompatibility complex (MHC)
molecules that mark cells as “self”
what is an antibody
molecules produced in response to an antigen
ex. immunoglobulins
what does it mean to opsonize
to coat an antigen with antibodies or complement proteins for enhanced immune response
what do complement proteins do
they help antibodies by:
* destroying bacteria
* inducing inflammatory responses
* regulating immune reactions
how does age affect the immune system
older individuals have a decreased response, while neonates have an underdeveloped immune system
what antigen characteristics determine immunogenicity
- molecular size
- foreignness
- chemical complexity
- ability to be processed and presented within MHC complex
how does antigen size affect immunogenicity
larger antigens have stronger immunogenicity
what are exceptions to the immunogenicity size rule
some small compounds can be immunogenic
what is foreignness in antigens
- how different an antigen is from the host
- the more unrelated an antigen is to the host, the more successful it is as an immunogen
What are the three types of antigens based on foreignness
- autoantigens (self)
*alloantigens (same species) - heteroantigens (different species)
*heterophile (unexpected cross reactions between species)
How does chemical complexity affect antigenicity
- proteins and polysaccharides are strong immunogens
- nucleic acids, lipids, synthetic polymers, and oligosaccharides are weak immunogens
How do carbohydrates function as immunogens
- less immunogenic than proteins
- cause T cell-independent responses
Why are lipids and nucleic acids poor immunogens
they are not immunogenic unless linked to an immunogenic carrier
can large molecules have multiple epitopes
Yes, each is capable of producing an immune response
What are the two types of epitopes
- linear Epitopes
- conformational epitopes
what is the difference between linear and conformational epitopes
- linear epitopes consist of sequential amino acids
ex. single polypeptide chain - conformational epitopes are one or more polypeptide chains
What cells recognize epitopes
B and T cells
What cell detects epitopes on the surface of a molecule
B cells
what epitopes do T cells detect
- linear epitopes that have been degraded
- Requires antigen to be presented by MHC
What is an antigenic determinant
another name for epitope
What is cross-reactivity
when an immune response to one molecule also reacts with a different molecule due to similar/shared epitopes
what is a hapten
a substance that is nonimmunogenic by itself but can form antigenic determinants when combined with a carrier
- nonimmunogenic by itself but immunogenic with carrier
what is a carrier molecule
a larger molecule that links with haptens to produce an immunogenic response
what is an adjuvant
a substance delivered with an antigen to enhance the immune response
*ex. adjuvants added to a vaccine
what increases the immunogenicity of an antigen by stimulating an innate immune response
an adjuvant
How do adjuvants affect the antigens location
they prevent the antigen from diffusing away from the injection site allowing immune cells to accumulate in the site.
* This tricks the immune system into acting as if there was an infection
what is an autoantigen
antigens that belong to the host and usually do not evoke an immune response
When would autoantigens invoke an immune response
when an autoimmune condition arises
what is an alloantigen
antigens that are from other members of the host’s species
- different blood types in humans
what is a heteroantigen
antigens from other species
*animal and human antigens are different
what is a heterophile
Heteroantigens are antigens found in different species, but their structures are so similar that antibodies made to one can also react with the other. In other words, if your immune system makes antibodies against an antigen from one species, those antibodies might also attack a similar antigen from another species.
What are the human MHC antigens called
human leukocyte antigens (HLAs)
What complex plays a pivotal role in the development of humoral and cellular immunity
the major histocompatibility complex (MHC)
where are the genes coding for MHC molecules located
they are found on the short arm of chromosome 6
how are MHC genes divided
They are divided into 3 classes:
*Class I
*Class II
*Class III
what do class I of MHC genes include
Class one is found on 3 different loci
*A
*B
*C
(A Big Cat)
what do class II of MHC genes include
Class II is found in the D region
* DP
* DQ
* DR
(Doesn’t Question Danger)
What do class III MHC genes include
Found in the middle of class I and II
- C4a
- C4b
- C2
- B
*cytokines
(Clever Cats Battle)
what is unique about the MHC
It is the most polymorphic gene system in humans
what is an allele
alternative forms of a gene that code for a slightly different variation of the same product
Where is MHC class one expressed
expressed on all nucleated cells
How does the MHC class I vary in expression
- More expressed on lymphocytes and myeloid cells
- less expressed on liver hepatocytes, neural cells, muscle cells, and sperm
What regions on chromosome 6 codes for MHC class I molecules
MHC class I is encoded by regions A, B, and C
(A Big Cat)
What is the function of MHC class I
antigen presentation of endogenous antigens
*Endogenous antigens: arise from the same cell
What is the structure of MHC class I
- Has a glycoprotein dimer
- alpha chain with 3 domains
- one beta-2 microglobulin
Why is MHC class I referred to as the endogenous pathway
Both peptides and MHC molecules arise within the same cell
How does MHC class I interact with antigens
MHC Class I molecules interact with endogenous antigens and present them to CD8+ cytotoxic T cells (Tc)
What happens when CD8+ Tc cells bind to the MHC Class I-antigen complex
CD8+ Tc cells release enzymes that destroy the target cell
Where is MHC class II found
found on antigen presenting cells (APCs) include:
* B lymphocytes
* monocytes
* macrophages
* dendritic cells
* thymic epithelial cells
What type of antigens does MHC Class II recognize
exogenous antigens
what is the function of MHC class II molecules
Presents exogenous antigens to CD4+ helper T cells
What happens after MHC Class II presents an antigen
CD4+ T helper cells stimulate B cells to divide and differentiate into antibody-producing plasma cells
Which MHC regions encode Class II molecules
- DP
- DQ
- DR
(Don’t Question Danger)
What is the structure of MHC Class II
- Made of two noncovalently bound polypeptides
- α chain with two domains
- β chain with two domains
why is MHC class II referred as the exogenous pathway
it processes and presents antigens that come from outside the cell
