Immunology Flashcards
(95 cards)
What are the 2 mechanisms of human defence?
Innate immunity
Adaptive immunity
Where do antibodies come from?
Synthesised and secreted by B lymphocytes
Before birth, they’re produced in the foetal liver
After birth- produced in the bone marrow from haematoitic stem cells
Outline the development of B cell sin the bone marrow
Bone marrow stroma provides the microenvironment for B lymphocytes to mature AND differentiate
Useful cells- exported to the periphery
Self- reactive B cells are disposed of
Ensures that each cell only has 1 specificity
Describe the T cell- independent responses
B cells (B-1) reside in the marginal zone of speed and are activated by polysaccharide and lipid antigens
Majority of B cells are follicular B cells (B-2) because they circulate through the follicles of lymphoid organs
Follicular B cells make the bulk of T-cell dependant class switches
They have a high-affinity antibody responses to protein antigens an give rise to long-lived plasma cells
Describe the 6 stages of B cell activation
- Membrane bound Ig recognises and binds to the microbe
- Receptor is non-covalently associated with 2 proteins: Ig alpha and beta
- Ig-alpha and Ig- beta contain conserved immunoreceptor tyrosine based activation motifs
- Signals initiated by binding to antigen receptor are transduced by recpetor-associated proteins
- Net result= Activation of transcription factors such as myc, NF-kb or AP-1
- Transcription factors switch on genes whose proteins are involved in cell proliferation and antibody synthesis
Describe the basic structure of an antibody
Glycoproteins
Range in size from 150kDa-900kDa
2x Heavy chains and 2x Light chains
N terminal= antigen binding site
C- terminal= Fc region
What are heavy chains?
Encoded by a single locus on human chromosome 14
Type of heavy chain present defines class of antibody
Distinct heavy chains differ in size and amino acid composition
What are light chains?
Encoded by 2 gene loci
Kappa locus is on human c-some 2
Lambda locus on human csome 22
Molecular weight of each light chain is approx 25kDa
Determine the specificity of antigen binding
2 identical light chains are either bases on either kappa or lambda
Describe the structure and function of immunoglobulins
Immunoglobulins are bifunctional proteins:
Thye must interact with a small number of specialised molecules- Fc receptors on cells- main classes include Fc alpha, Fc. , Fc gamma
They complement proteins
They’re intracellular cell signalling molecules whilst recognising an infinite array of antigenic determinants at the same time
Cell surface antigen receptor of B cells- Allows b cells to sense their antigenic environment
Secreted antibody- Neutralises toxins or microbes
Used for agglutination of viruses or immune complexes
Used to arm or recruit effector cells
Describe immunoglobulin G
Most abundant serum antibody and makes aprox 80% of total serum antibodies
Consists of 2 heavy chains and either 2 kappa/ lambda light changing
4 subclasses: 1-4
Longest half-life of all antibodies- aprox 23 days
What are the functions of IgG?
Major antibody of secondary immune system
IgG 3 &4 readily cross the placenta= important roles in the neonatal protection
IgG3 is the most effective activator of compliment
IgG4 does activate complement
IgG1/3 binds wiht a high affinity to Fc receptors on phagocytotic cells and thus mediates opsonisation
IgG aids immobilising bacterial threats and neutralises toxins and viruses
Describe immunoglobulin M
Largest Ig
Memebrane bound form is ALWAYS monomeric
Aprox 1000kDa
Found intravascularly
Aprox 10% of plasma Ig’s
Describe immunoglobulin A
Aprox 15% of plasma Ig
Major tissue Ig found lining mucosal surfaces: GI tract, Respiratory tract and Urogenital tract
The SC protect IgA from degradation by gastric acid and digestive proteolytic enzymes
Often found as a dimer
Key in neutralising antibody in the mucosal compartment
Exists in two subclasses and differs between 2 compartments where it is found- blood and mucosal secretions
Blood IgA is monomeric
Mucosal IgA is almost always dimeric
Describe immunoglobulin D
180kDA
Delta H chains= 70 kDa
Rarely detected in blood
Two classes: 1 and 2
Plays an important role in maturation and proliferation of B cells
Describe immunoglobulin E
200kDa
Epilson chains of approx 73 kDa
IgE constitutes 0.3% of serum Ig
Major immunoglobulins in mediating allergic and anti-parasitic responses
Cross-link to the FC epilsonR on mast cells
Triggers the release of histamine, tryptase and proteolytic enzymes by mast cells
How do antibodies interfere with HIV?
A. Virus- target cell interaction blocked
B. Post attachment stages blocked
C. Virus- cell fusion inhibited
D. Virus uncoating prevented, viral assembly and maturation interrupted and virus budding inhibited
E. Infectious virions aggregated
What are the stages of complement action?
- Pattern recognition acts as trigger
- Protease cascade amplified
- Inflammation, phagocytosis or membrane attack
What is RAG1 and RAG-2?
2 DN shuffling enzymes that are encoded by recombination activating genes aka V(D)J
B and T cells express enzymes here during development and maturation
They initiate the molecular processes that lead to lymphocyte receptor formation by V(D)J segment recombination
Nonsense mutation in RA1/2 cause the most profound immunodeficiency, sever recombined immunodeficiency (SCID)
Describe central tolerance
The processes of eliminating and developing T/B lymphocytes that are auto reactive
Bone marrow stromal cells and blood plasma provide ncomplete encyclopaedia of self antigens to train B cells
Any B cell that binds strongly to these self antigens are either deleted/ recycled
What is peripheral tolerance?
When auto reactive B cell are released into peripheral lymphoid organs and they’re silenced by several mechanisms
A major mechanism being- B anergy in which auto reactive B cells are maintained in an antigen unresponsive state
Describe the principles of cellular co-operation
Antibody production is improved when T cells are present
“Purified” lymphocytes were shown to produce immune responses in vitro to antigens
Contaminated with macrophages, when truly purified with little response, therefore macrophages are important- seen to have another role in antigen presentation
What is MHC?
MAJOR HISTOCOMBATABIITY COMPLEX= Special structre cells used to present antigen to lymphocytes
B cells recognise antigen without MHC- surface bound antibody referred to as B cell Receptor
These can act as antigen presenting cells (APCS)
T cells can only “see” antigen presented on MHC
What are T cells?
Lymphoid cels of adaptive immune system
Formed from lymphoid stem cells, produced in the bone marrow
Mature in the thymus where they produce a T cell receptor (TCR)
2 main subsets of T cells:
1. Helper T cells- Produce proteins to act as messengers for the rest of the immune system, in the form of cytokines
2. Killer T cells- cause infected cells to apotose
Once triggered hey can kill a cell in an antigen specific manner
A 3rd subset has been characterised recently: Regulatory T cells= Keep other T cells under control
What are APC’s?
Antigen presenting cells
Professional APCs deliver antigen to CD4+ T cells
Dendritic cells, macrophages and B cells
All APCs require class II MHC molecules in addition to other co-stimulatory molecules on their surface