4 Principles of the immune system
- Detect, respond and eliminate pathogens
- Maintain tolerance to self antigens, environmental antigens and in pregnancy to paternal antigens
- Induce memory (rapid greater response) to previously encountered infections
- Restore organ/tissue homeostasis
2 broad responses of the immune system in response to a pathogen
- Innate immune responses
- Adaptive immune responses
Outline the cells which form part of the innate immune response
Epithelial cells
Granulocytes
Mast cells
Tissue macrophages
Sensory Neurons
Complement
Dendritic cells
Outline the actions of the innate immune response
- Barrier function
- Secrete cytokines, chemokines and interferons
- activate complement response
- Recruit circulating neutrophils and monocytes for internal (phagosome-lysosome formation) and external (cell degranualtion) pathogen killing
- Induce adaptive immune responses in secondary Lymphoid Tissue (dendritic cell phagocytoses pathogen > become mature and move to secondary LT > present antigens to prime lymphocytes)
What cell delineates the transition of the innate to adaptive immune response and outline the 2 ways it impacts the adaptive immune response
Dendritic Cell
- Present processed antigen via MHC II to CD4 T-cells in lymph nodes to prime the adaptive immune response
- Express cytokines to attract specific type of CD4 T cell and therefore the type of adaptive immune response. The cytokines which they express varies depending on the signal of the pathogen classe their Pathogen-Recognition Receptors and Danger receptors pick up.
Briefly outline the different typess of adaptive immune responses to infection
Type 1 = against intracellular pathogens
Type 2 = against extracellular parasites
**Type 3 **= against extracellular bacteria and fungi
Describe the pathway of type 1 immune response
include: main immune cell and cytokine
Main immune cell = CD4 Th1 Cell
Main cytokine = IFN gamma
(right side of diagram = in lymph nodes)
Describe the pathway of type 2 immune response
include: main immune cell and cytokine
Describe the pathway of type 3 immune response
include: main immune cell and cytokine
Outline the 4 main types of B Cells
Innate-like B cell
* spontaneous production of IgM
Marginal Zone B Cell
* T-Cell independent immune responses - hence do not need to T-cell activation to begin producting antibodies
* Located outside germinal centres in lymphoid organs (commonly found in spleen and GI tissue)
* Protection against encapsulated organisms (commonly carbohydrate - e.g. pneumococcus)
* Produce IgM then IgA then IgG
* Have limited memory and antibody affinity
* Those that have had splenectomy and therefore lack Marginal Zone B Cells are at higher risk of overwhelming infection of encapsulated organisms
Extra-Follicular B Cells
* Respond to proteins (e.g. antigens)
* T-cell dependent and independent
* Rapid response (24-96 hours)
* Can undergo class switching of immunoglobulins (low affinity)
* Produce short lived plasma cells (memory - e.g. sars-COV2 response is by extra-follicular b cells and hence why ppl need booster vaccines as memory is short-lived)
Germinal Centre B Cells
* Respond to proteins (e.g. antigens)
* T-cell dependent activation
* 5-7 days (slower-onset of action)
* Undergo class switching
* Memory - long-term as gives rise to long-lived plasma cells
* High affinity antibodies
Outline the structure of immunoglobulins
2 heavy chains + 2 light chains
2 functional units:
* Fab (fragment-antigen binding) unit - binds to specific antigen
* Fc (constant fragment) unit - undergoes class switching and therefore depending on Ig class there is a different immune response.
Function of the different classes of immunoglobulins
IgM
* antibody produced in primary response
* less affinity
* important to activate complement immune action
IgG
* Most abundant class - has 4 isotypes
* Neutralises pathogen by opsonization (allows phagocytes to phagocytose pathogen where before they evaded phagocytosis)
IgE
* Defence against parasite infections
* Associated with hypersensitivity
* Activation of mast cells and basophils
IgA
* predominantly found in secretions and mucosa
* binds to bacteria neutralising it (by binding to it, it prevents bacteria to invade through epithelium and adhere to cells to cause damage and also allows easier for phagocytosis and excretion)
IgD
* action not yet known
Define inborn errors of immunity (IEL)
Heterogenous group of genetic disorders resulting in immune dysfunction
What type of mutations cause IELs
Single gene mutations - approximately 500 single gene mutations have been identified.
What are the commonest IELs
Antibody deficiency (hence cellular component behind most IELs is B cell)
B and T cell dysfunction
Complement deficiencies
Concequences of IELs
- Increased susceptibility to infection
- Autoimmune disease (e.g. SCID or very early onset of other autoimmune diseases)
- Allergic disease
- Auto-inflammatory disease
- Increased risk of viral related cancers (EBV, HPV)
Tell-tale signs of patients with underlying IELs
Infections: SPUR
- Severe (resulting in sepsis)
- Persistent (multple course of abx required to treat chest infecitons etc)
- Unusual (opportunisitc organisms e.g. pneumocystitis jjrovecci)
- Recurrent (more than 2 epsiodes of pneumonia in 1 year or 8 epsidoes of otitis media in a child)
Early onset or resistant autoimmune cytopenias
Very early onset inflammatory disease (IBD, inflammatory skin disease)
Difficult to treat allergic skin disease with systemic features to suggest IEL (recurrent infections, autoimmune disease)
Unexplained viral induced cancers in patients less than 40 years
Family history of immune deficiency or consanguinity
note that not all exisit simultaneously!
Describe the phenotype of infections patients with IELs are at risk of
Patients with IELs exhibit a highly variable infection phenotype. They can present with either:
- Having multiple episodes involving a wide range of infections (common organisms as well as rare opportunistic infections, infections following live vaccines). More common in children
- Susceptibility to weakly virulent organsims only (environmental TB species or BCG vaccine)
- Highly susceptibility to single organims only infections (e.g. invasinve Neisseria in complement pathway defects or HSV in neuronal defects)
What are the differences between autoinflammatory and autoimmune disorders
Autoinflammatory: hyperactivation of the innate immune system
Autoimmune: abnormalities of the adaptive immune system
Why do autoimmune disease arise from IELs
Autoimmune diseases arise from IELs as a result of inborn errors of:
* T cell tolerance
* T cell apoptosis
* T regulatory function
Therefore resul in presence of pathological self reactive T cell immune responses
Describe the autoimmune disease phenotype in patients with IELs
- SCID or Hypomorphic SCID
- Early onset autoimmune disease (SLE, ITP, AIHA) - note that childhood onset of these diseases is indiciative of IELs!
- Difficult to treat autoimmune cytopaenia
Why do autoinflammatory disease arise from IELs
Due to aberrant activation of innate inflammatory pathways in the absence of antigen directed autoimmunity
Describe the autoinflammatory disease phenotype in patients with IELs
Clinical presentation: fever, skin rashes, arthritis, other manifestations of inflammatory disease in the absence of autoimmunity or infection
2 main categories of monogenic (single gene) autoinflammatory disorders:
* IL-1inflammasomopathies (Familial Mediterranaen fever)
* Type 1 Interferonopathies (Aicardi Goutiers Syndrome)
What are the immune defect that results in allergic disorders in patients with IELs
- Failure of CD4 Th17 development (oral candida)
- Defect IL-6 signalling (staph a infeciton, cutaneous allergic disease, increased IgE)
