book 8 Flashcards
1
Q
state the role of antigen-presenting cells
A
an APC is involved in antigen presentation whereby they contain surface receptors that recognises common features of pathogens. the binding of APC receptor to pathogen stimulates the APC to engulf he pathogen and degrade it inside the cell. the antigens are then processed and then displayed on major histocompatibility complexes (MHC) class II molecules, so that T cells may then recognise the antigen using the specific T cell receptors
2
Q
describe the role of CD4 T cells
A
- after APC engulfs and degrades a pathogen, it displays antigen fragments on class II MHC molecules
- a specific CD4 T cell then binds to the antigen via the unique T cell receptor, this interaction promotes the secretion of cytokines by the APC
- to activate CD4 T cells, a second interaction between CD28 protein on the CD4 T cells and the CD80/CD86 protein on the APC
- stimulated by cytokines, the specific T cell undergoes proliferation and differentiation, giving rise to many clones of activated helper T cells and memory helper T cells. these cells have the same T cell receptors that are complementary in 3D conformation to the same MHC-antigen fragment complex – this process is called clonal selection and expansion
- after the infection, most of the effector T cells die but memory T cells persist in the body for a long time
- T cell receptors of memory T cells will have a higher affinity for the same antigen. when the same antigen is encountered in the future, the memory helper T cells will bind more readily to it and proliferate and differentiate faster into effector cells that quickly activate the specific B cells and the specific cytotoxic T cells to illicit the secondary immune response
3
Q
state the roles of helper T cells
A
- they are required to release cytokines that results in the proliferation and differentiation of specific CD8 T cells into effector cytotoxic T cells and memory cytotoxic T cells
- helper T cell also binds to antigens presented by specific B cells and release cytokines that results in the proliferation and differentiation of specific B cells into antibody-producing plasma cells and memory B cells
4
Q
describe the role of CD8 T cells and their activation process
A
- fragments of foreign proteins produced in infected host cells are presented on the MHC class I molecules and are displayed on the cell surface, where they can be recognised and bound by specific CD8 T cell receptors
- helper T cells in the vicinity releases cytokines
- both interactions results in the activation of specific CD8 T cells
- activated specific CD8 T cells undergo proliferation and differentiation, giving rise to many clones of activated cytotoxic T cells and memory cytotoxic T cells which all have the same T cell receptor that are complementary in 3D conformation to the same MHC-antigen fragment complex that the original CD8 T cell bound to – this process is called clonal selection and expansion
- after the infection, most of the effector T cell die but memory cytotoxic T cells persists for a long time in the body. the T cell receptor of such memory cytotoxic T cells have higher affinity for the same antigen
- when the same antigen is encountered in the future, these memory cytotoxic T cell binds more readily to it and proliferate and differentiate faster into effector cells to illicit secondary immune response.
5
Q
state the role of cytotoxic T cells
A
- when bound to infected cells that present the same antigen, the activated cytotoxic T cells then kills the infected cell via:
1. it secretes perforin which forms pores in the infected cell membrane and disrupt membrane integrity
2. it also releases granzymes that break down proteins. granzymes enter the infected cell by endocytosis and trigger cell death via apoptosis - after the death of infected cell, the activated cytotoxic T cell is released to recognise and destroy other infected cells presenting the same antigen
6
Q
describe the roll of B cells and their activation process
A
- the antigen on the pathogen binds to the B cell receptor of a specific B cell, by complementary 3D conformation. the B cell engulfs the pathogen, degrades it using hydrolytic enzymes of lysosomes, processes it and display it on the antigen fragment MHC class II molecule
- antigen fragment displayed on the MHC class II molecule on the B cell binds to an activated T helper cells which have receptors that are specific for the antigen fragment
- cytokines are released by the helper T cell to activate the B cell
- specific activated B cell then undergoes proliferation and differentiation to give rise to many clones of activated plasma cells and memory B cells. these memory B cells have the same B cell receptors that are complementary in 3D conformation to the same antigen
- plasma cells produce and secrete antibodies with antigen binding sites that have the same 3D conformation as the specific B cell receptors – they can bind to the same antigen that original B cell bound to. this process is clonal selection and expansion
- memory B cells persist for a long time in the body and receptors of memory B cells will have higher affinity for the same antigen. when the same antigen is encountered in the future, the memory B cells bind more readily to it and proliferate and differentiate faster into effector cell to illicit secondary immune response
7
Q
state the role of plasma cells
A
- plasma cells produce and secrete antibodies of the same conformation as the B cell receptors.
- antibodies functions to:
1. bind to specific foreign antigens on pathogens and neutralize them
2. recruit phagocytes for engulfment - opsonization
8
Q
describe the structure of antibodies
A
antibodies are glycoproteins consisting of 4 polypeptide chains (quartenary structure) : 2 identical heavy chains and 2 identical light chains
9
Q
state the function of variable region
A
the variable region determines the 3D conformation of the antigen-binding site and hence the specificity of the antibody
10
Q
state the function of having 2 identical antigen-binding sites
A
this allows antibody molecules to cross-link with the antigens and to ensure that the binding is stable
11
Q
state the function of a constant region
A
it determines the class of antibodies and allows the binding to receptors on phagocytes
12
Q
explain how somatic recombination results in different antibody molecules
A
- somatic recombination occurs during the B cell development, at the DNA level of both light and heavy chain genes
- for the light chain gene, only one segment V and one segment J are first arranged together, followed by the C segment
- for the heavy chain gene, only one D segment and one J segment are first arranged together, followed by one V segment being arranged next to the pre-arranged DJ region, along with the C segment
- after transcription, a pre-mRNA is formed which contains
- the V J C segments as well as introns for the light chains
- the V D J C segments as well as the introns for the heavy chain - the pre-mRNA of the light chain is processed by mRNA splicing in the nucleus to remove the introns, a 5’ cap and 3’ poly-A-tail are also added. the heavy chain is processed by mRNA splicing in the nucleus to remove the introns. a 3’ poly-A-tail is also added
- translation results in formation of light and heavy chain polypeptide consisting of variable region and a constant region
- different light and heavy chain variable regions associate to form different antigen binding sites of the antibody
- the outcome is increased antibody diversity
13
Q
explain how somatic hypermutation leads to millions of different antibody molecules
A
- somatic hypermutation occurs in activated B cell which were activated by helper T cell after encountering an antigen
- somatic hypermutation introduces frequent point mutations into the variable regions/ V, D and J segments of the already rearranged heavy and light chain genes at a high rate
- this gives rise to slightly different light and heavy chain variable regions
- the outcome is to allow for secretion of antibodies that can bind better to the same antigen
14
Q
explain how class switching leads to millions of different antibody molecules
A
- class switching occurs in activated B cell and in memory B cell
- during the course of an antibody response, activated B cells can switch to the expression of a different class of antibody via class switching
- the heavy chain constant regions determine the class of the antibody. they are encoded by separate gene segments of the heavy chain gene
- during class switching, different C segments coding for different heavy chain constant regions are re-arranged and then joined to the previously re-arranged V, D or J segments, done in somatic recombination, coding for the same variable region
- after transcription and translation, different heavy chain constant regions are made and then linked to the same variable region
- class switching is stimulated by external signals such as cytokines released by T cells or signals delivered by pathogens
- the outcome is that class switching allows for the production of different classes of antibodies at different times of the immune response. different classes of antibodies bind to the same antigen but will evoke a different effector function
