IMMUNO: Primary Immunodeficiency

Question 1

List the categories of autoimmune disease.

Answer 1

Overactivation:

• Auto-inflammatory disease
• Autoimmune disease
• Allergic disease

Underactivation:

• Primary immunodeficiency
• Secondary immunodeficiency

Question 2

How are immunodeficiencies classified?

Answer 2

1. Primary - inherited - clinically important immunodeficienciesare rare (1:10,000 live births)
2. Secondary - acquired - infection, malignancy, drugs, nutritional deficiencies (common)
3. Physiological

• Neonates
• Pregnancy
• Older age

Question 3

What clinical features are suggestive of immunodeficiency?

Answer 3

1. Two major or one major and recurrent minor infections in one year
2. Atypical organisms, unusual sites, poor response to treatment
3. Primary immune deficiency in the family, presenting at young age, failure to thrive

Question 4

What can primary immunodeficiency be split into and refrence the cells/soluble components involved ?

Answer 4

1. Innate system
   
   • Macrophages + Granulocytes (eosinophis, basophils, neutrophils)
   • Dendritic cells (APP)
   • Cytokines and receptors
   • NK cells (note: derived from lymphoid progenitor)
2. Adaptive system
   
   • T-Cells
   • B-cells
3. Complement system

NOTE: Can be immunodeficient across all arms of the immune response

Question 5

Summary of phagocyte deficiency immunodeficiency.

Question 6

What are the four types of phagocyte deficiency?

Answer 6

1. Failure to produce neutrophils - Deficiency within the bone marrow
2. Defect of phagocyte migration - Neutrophils fail to migrate out of the blood
3. Failure of oxidative killing mechanisms = Chronic Granulomatous Disease-
4. Cytokine deficiency = Failure of T-cell activation

Question 7

Deficiency within the bone marrow

List 3 types of failure to produce neutrophils. State which are autosomal recessive/dominant + mutaions responsible?

Answer 7

Deficiency within the bone marrow

• Reticular Dysgenesis- failure to get differentiation of stem cells down ANY of the lineages
  
  • Autosomal recessive, severe SCID
  • Mutation in mitochondrial energy metabolism enzyme adenylate kinase 2 (AK2)
• So DO NOT have any myeloid (inc macrophages, granulocytes) or lymphoid cells
• This is a catastrophic form of severe combined immunodeficiency affecting multiple cell lines

Severe Congenital Neutropaenia (Kostmann syndrome)- a neutrophil-specific deficiency

• Classical form due to mutation in HCLS1-associated protein X-1 (HAX-1)

Cyclic neutropaenia - episodic neutropenia every 4-6 weeks

• Mutation in neutrophil elastase (ELA-2)

Question 8

Name a defect of phagocyte migration, explain MOA and findings on investigations.

Answer 8

Leukocyte adhesion deficiency -Leukocyte adhesion deficiency -where neutrophils are recruited to site of inflammation/ injury but fail to migrate out of the blood due to deficiency in leukocyte adhesion molecules (specifically CD18): CD18 (β2 integrin subunit) binds to CD11a forming a molecule called LFA-1, which is expressed on neutrophils. This binds to the ligand (ICAM-1) on endothelial cells and so regulates neutrophil adhesion/ transmigration

In LAD, the neutrophils lack these adhesion molecules and FAIL to exit from the blood stream

• Very high neutrophil counts in the blood
• ABSENCE of pus formation

(Get lots of neutrophils in the blood stream but cannot get into the periphery)

Question 9

What are neutrophil levels in leukocyte adhesion deficiency?

Answer 9

Very high in bloodstream (because cannot exit into tissues)

No pus formation

Question 10

Name disease caused by failure of oxdative killing mechanims in neutrophil-mediate immunodeficiency. What is the pathophysiology of this?

Answer 10

Chronic granulomatous disease

No respiratory burst due to deficiency of one or components of NADPH oxidase –> unable to generate Reactive Oxygen Species –> impaired killing –> neutrophils/macrophages accumulate causing excessive inflammation –> granuloma formation and lymphadenopathy/hepatosplenomegaly

For added understanding ( donb’t need to learn)

There are lots of macrophages and migrated neutrophils in the periphery. This is where they see the pathogens and phagocytosis occurs via oxidative killing

Chronic Granulomatous Disease- Failure of oxidative killing due to deficiency of NADPH oxidase

So, oxidative species are not formed and cannot kill pathogens well

Question 11

What is seen in Chronic granulomatous disease + Name a specific treatment that can be used ?

Answer 11

Deficiency of one of the components of NADPH oxidase leads to an inability to generate oxygen free radicals, resulting in impaired killing

Phagocytes will try and phagocytose the pathogen but will not be successful

• Excessive inflammation
• Persistent neutrophil/ macrophage accumulation
• Failure to degrade antigens
• Granuloma formation
• Lymphadenopathy and hepatosplenomegaly

Treatment: Interferon gamma

Question 12

Name and describe two investigations for chronic granulomatous disease.

Answer 12

First step is to activate the neutrophils which should have a respiratory burst and produce hydrogen peroxide

Nitroblue terazolium (NBT) test - NBT is a dye which changes colour yellow to blue if it interacts with hydrogen peroxide

Dihydrorhodamine (DHR) flow cytometry test - DHR is oxidised to rhodamine which is fluorescent after hydrogen peroxide

Question 13

Name a type of cytokine deficiency which can cause phagocyte-mediated immunodeficiency and explain mOA.

Answer 13

IFNγ, IFNγR, IL-12 or IL-12R Deficiency = failure of cytokine production and subsequent T cell activation. This results in the inability to form granulomas

1. After oxidative killing, neutrophils die and form pus, whilst macrophages persist and present antigens to T cells (a macrophage-T cell interaction)
2. Activated macrophages produce IL-12
3. IL-12 stimulates T cells to produce IFN-gamma
4. IFN-gamma feeds back to neutrophils and macrophages to stimulate:
   
   • TNF production
   • NADPH oxidase activation
5. Oxidative pathways are then stimulated

Question 14

What are the clinical complications of phagocyte deficiency ?

Answer 14

1. Recurrent bacterial/fungal infections –skin / mouth

• Bacterial infections: catalase positive bacteria i.e. PLACES (Pseduomonas, Listeria, Aspergillus, Candida, E.Coli, Staph Aureus)
  
  • Staphylococcus aureus
  • Enteric bacteria
• Fungal infections
  
  • Candida albicans
  • Aspergillus fumigatus and flavus

2. Mycobacterial infection

• Mycobacterium tuberculosis
• Atypical Mycobacteria

Question 15

Which types of infections are common in cytokine deficiency mediated phagocyte deficiency (particularly IL-12 deficiency)?

Answer 15

IL-12- IFNγ network deficiency causes mycobactrial infection

*

Question 16

What is the management of pahgocyte deficinecy immunodeficiency?

Answer 16

Aggressive management of infection

• Infection prophylaxis - prophylactic antibiotics anti-fungals
• Treatment with oral/IV antibiotics as needed

Definitive therapy

• Haematopoietic stem cell transplantation - replaces the defective population
• Specific treatments: IFN-gamma therapy for Chronic Granulomatous Disease

Question 17

What is the function of NK cells and how do they become activated ?

Answer 17

They are cytotoxic cells that Migrate from blood into inflamed tissues

Have an activatory receptor including cytotoxicity receptors which recognise heparan sulfate proteoglycans

Have an inhibitory receptor which recognised HLA class I to prevent inappropriate activation

• Malignant/virus infected cells downregulate HLA class I = NO natural inhibitory influence on the NK cell, leading to activation of the activating receptor and cytotoxicity and production of cytokines

So NK cells can be activated directly by the pathogen recognising heparan sulphate proteoglycans or indirectly due to down regulation of HLA → loss of inhibitory signals.

Question 18

Name and decribe two types of NK deficiencies.

Answer 18

1. Classical NK deficiency
   
   • Absence of NK cells within peripheral blood
2. Functional NK deficiency
   
   • NK cells present but function is abnormal

Question 19

Which infections are most commonly associated with NK cell deficiencies?

Answer 19

1. Intracellular virus infections e.g. Human Herpes Virus infections [e.g HSV I and II, VZV- may be disseminated, EBV, CMV]
2. HPV - including malignancy (HPV associated cancers)

Question 20

What is the management for NK cell deficiencies?

Answer 20

1. Prophylactic antivirals e.g. acyclovir or gancyclovir
2. Cytokines such as IFN-alpha to stimulate NK cytotoxic function
3. HPSC transplantation if severe

Question 21

Match these up.

Answer 21

1. leukocyte adhesion deficiency
2. CGD
3. Kostmann syndrome
4. IFN-gamma R deficiency
5. Classical NK cell deficiency

Question 22

Where is complement produced and how many components are there and what happens when activatd?

Answer 22

>20 tighly regulated, linked proteins produced in the liver + present in the circulation as inactive molecules

When triggered, enzymatically activate other proteins in a biological cascade. Results in a rapid, highly amplified response

Question 23

Draw the complement cascade.

Answer 23

Question 24

What activates the classical pathway?

Answer 24

1. Activated by formation of antibody-antigen immune complexes
2. Contact results in change in antibody shape, exposing the binding site for C1
3. C1 binds to the antibody site which causes activation of the cascade involving C1, C2 and C4 → come down onto C3

Dependent upon activation of acquired immune response (antibody) before activating this arm of complement so it is a bit slow

Need immune complexes to activate antigen-binding to the antibody which subsequently exposes the site for C1q to bind.

Question 25

What are the components of the classical pathway?

Answer 25

C1, C2, C4 --\> C3 ---\> C5-C9 (final common pathway) --\> membrane attack complex

Question 26

What activates the mannose binding lectin pathway? What is its advantage over classical?

Answer 26

1. Activated by direct binding of MBL to microbial cell surface carbohydrates 2. This stimulates the classical pathway C2 and C4 (NOT C1) --\> C3 ---\> C5-9 ## Footnote *Not dependent on the acquired immune response*

Question 27

How many MBL deficiency present?

Answer 27

MBL deficiency = recurrent infections when neutropaenic following chemotherapy but previously well (i.e NOT usually associated with immunodeficiency - this is a normal response) i.e Associated with increased infection in patients who have another cause of immune impairment

Question 28

What activates the alternate pathway? What factors are involved? Which is the control protein?

Answer 28

1. Activated by bacterial cell wall components e.g. lipopoolysaccharise of **gram negative** bacteria, teichoic acid of **gram positive** bacteria. 2. Involves **factors B, D** and **properidin**. 3. **Factor H** is the control protein (usually -ve regulated to regulate spontaneous activation) ## Footnote *Not dependent on the acquired immune response.*

Question 29

Final Common Pathway - what factors involved and what does it do?

Answer 29

Pathways Converge on Activation of C3 * Activation of C3 is the major amplification step in the complement cascade * It triggers the formation of the membrane attack complex via C5-9 [Final Common Pathway] * [Final Common Pathway] leads to the formation of the membrane attack complex (which attacks the membrane and therefore kills)

Question 30

What is the function of activation of C3?

Answer 30

Triggers formation of the membrane attack complex via C5-9 ## Footnote *MAC causes membrane lesions*

Question 31

List 6 other immmune functions of complement in addition to formation of the membrane attack complex via final common pathway

Answer 31

1. Increases vascular permeability and cell trafficking to site of inflammation 2. Promotes clearance of immune complexes so they do NOT deposit in blood vessels and cause inflammation 3. Activates phagocytes 4. Opsonisation of pathogens to promote phagocytosis 5. Promotes mast cell/ basophil degranulation 6. Punches holes in bacterial membranes

Question 32

What bacterial infections are those with complement deficiency most susceptible to?

Answer 32

Especially those by encapsulated bacteria (NHS): * Neisseria meningitides (esp in properidin and C5-9 deficiency) * Haemophilus influenzae * Streptococcus pneumoniae The final common pathway is VERY important for generating the MAC for the encapsulated bacteria like meningococcus. Note: MBL deficiency are common but are NOT usually associated with immunodeficiency - Associated with increased infection in patients who have another cause of immune impairment

Question 33

What are the results of C5-9 and properidin deficiency?

Answer 33

Susceptibility to Neisseria meningitides

Question 34

What are the results of C1, C2, C4 deficiency + explain pathophysiology + what ahpuld the levels of C3+C4 be?

Answer 34

Susceptibility to SLE because... 1. Classical complement pathway activation promotes phagocyte mediated clearance of --\> **_dead cells and nuclear debris_** * Deficiencies results in increased load of self-antigens, particularly nuclear components ∴ more likely to develop autoantibodies to some of this nuclear debris * This may promote autoimmunity and formation of immune complexes 2. Classical complement pathway activation promotes **_clearance of immune complexes_** (by erythrocytes??) * Complement fragments help to **_solubilise immune complexes_** and clear them * Deficiencies result in **_deposition of immune complexes_** which stimulation local inflammation in **_skin, joints and kidneys (SLE)_** * More likely to get consequences- **_rash, arthritis, nephritis_** NOTE:Normal C3 and C4 is odd in SLE as in SLE, SLE = increased immune complexes which should = Persistent activation of the classical pathway = low C3 and C4. active SLE but NORMAL levels of C3 and C4 indicates deficiency in C1, as classcial pathway can't be activated but normally levels sgould be depeleted

Question 35

3 things - Clinical Phenotype of Complement Deficiency of the early classical pathway and what is the most common?

Answer 35

Question 36

What are the consequences of MBL pathway deficiency?

Answer 36

MBL2 mutations are common but NOT USUALLY ASSOCIATED WITH IMMUNODEFICIENCY

Question 37

How does SLE contribute to immunodeficiency?

Answer 37

Persistent activation of the classical pathway leads to immune complex deposition and therefore low C3 and C4 NOTE:Normal C3 and C4 is odd in SLE as in SLE, SLE = increased immune complexes which should = Persistent activation of the classical pathway = low C3 and C4. active SLE but NORMAL levels of C3 and C4 indicates deficiency in C1, as classcial pathway can't be activated but normally levels sgould be depeleted

Question 38

What is the name for auto-antibodies against components of the complement pathway?

Answer 38

Nephritic factors = auto-antibodies against components of the complement pathway

Question 39

What are the effects of C3 nephritic factors on the complement cascade and what will you expect the levels of C3 + C4 to be?

Answer 39

Nephritic factors are auto-antibodies directed against components of the complement pathway * Nephritic factors stabilise C3 convertases, resulting in constant C3 activation and consumption * So, this depletes C3 * Often associated with **glomerulonephritis** (classicaly membranoproliferative) * Also associated with **partial lipodystrophy** (fat lost from upper body) low C3 with a NORMAL C4 - Ask for a nephritic factor

Question 40

How can auto-immune disease lead to complement deficiency?

Answer 40

1. SLE - leads to overconsumption of C3, C4 2. Nephritic factors - anti-C3 convertase so C3 overactivation leads to low C3 (C4 normnal) ## Footnote *NB: C3 and C4 are routinely measured in blood tests*

Question 41

Investigation of the Complement Pathway: What are two functional complement tests and other investigations for complement deficiency?

Answer 41

Functional complement tests **CH50** - classical pathway (testing activity of C1, C2, C4 and C5-9) **AP50** - alternate pathway (testing activity of B, D, Properidin and C5-9) NOTE: if both are abnormal, there will be an abnormality in C3 or C5-9. It will most likely be from C5-9 Investigation of the Complement Pathway Complement Assays ( to measure levels) : Complement C3 and C4

Question 42

Fill in the blanks for C3/4 levels and CH50 and AP50 test results for each complement deficiency.

Answer 42

* C1q deficiency leads to abnormal CH50 because classical pathway is not activated in the right way but normal AP50 because still activation alternate.* * Properidin is involved in the alternate pathway.* * C9 is part of classical and alternate as it is at the end.*

Question 43

How do you manage patients with complement deficiencies?

Answer 43

* Vaccination - **pneumovax, meningovax, HIB vaccine**s especially * Boost protection mediated by other arms of the immune system * Prophylactic antibiotics * Treat infections aggressively * Screen family members Screening of family members due to risk of meningitis and other serious infections

Question 44

Match up the answers.

Answer 44

1 - 1 2 - 2 3 - 4

Question 45

What is a primary lymphoid organ? Where are B and T cells formed and where do they mature?

Answer 45

Primary lymphoid organ - organs involved in lymphocyte development **B cells** - develop and mature in BM **T cells** - develop in BM and mature in thymus

Question 46

What is severe combined immunodeficiencies (SCID) and what is the most severe form?

Answer 46

Most severe form: reticular dysgenesis: Fatal in early life unless corrcted with BM transplantation

Question 47

What is the most common SCID?

Answer 47

*SCID because it is combined B and T cell deficiency.* X linked SCID - makes up 45% of all SCIDs

Question 48

What is the mutation in X linked SCID?

Answer 48

**Mutation of common gamma chain on** Chromosome X **-** The common γ chain is shared by a number of interleukin receptors = Inability to respond to cytokines causes: 1. early arrest of T and NK cell development and production of mmature B cells

Question 49

What are the lymphocyte levels in X-linked SCID vs ADA SCID ?

Answer 49

Inability to respond to cytokines causes early arrest of T cells, and NK cell development and production of immature B cells = Phenotype * Very low or absent T cell numbers * Very low or absent NK cell numbers * **Normal or increased B cell numbers** but l**ow Immunoglobulins** Phenotype * Very low or absent T cell numbers * Very low or absent NK cell numbers * **Very low or absent B cell numbers**

Question 50

What is the pathogenesis and phenotype of ADA deficiency??

Answer 50

16. 5% of all SCID * **Adenosine Deaminase Deficiency** - enzyme required for cell metabolism in lymphocytes **Phenotype** Inability to respond to cytokines causes early arrest of T cells, and NK cell development and production of immature B cells * Very low or absent T cell numbers * **Very low or absent B cell numbers** * Very low or absent NK cell numbers

Question 51

What protects the neonate in the first 3 months of life in SCID?

Answer 51

* Maternal source of circulating IgG in the neonate * The baby is born with lots of maternal IgG antibodies that selectively cross the placenta * SCIDs generally do well in the first 3 months * Neonates get more IgG from early breastmilk too * Then normally baby starts to produce their own IgG BUT if you don’t have your own IgG, start to get infections

Question 52

What are the (6) clinical features of SCID?

Answer 52

* Unwell by 3 months of age * Infections of all types * Failure to thrive * Unusual skin disease - **colonisation of infant’s empty bone marrow** by maternal lymphocytes and **graft versus host disease** * Family history of early infant death

Question 53

What is the difference in function of CD4+ and CD8+ T cells?

Answer 53

HLA Class I- becoming CD8 T cells (presentation of intracellular peptides) HLA Class II- becoming CD4 T cells(presentation of extracellular peptides)

Question 54

Describe the process of central tolerance and selection in T cell maturation.

Answer 54

1. Only T cells which have intermediate affinity for HLA are positively selected (10%) - *those with low affinity for HLA are not selected, and high affinity are negatively selected to avoid autoreactivity.* 2. Those with intermediate affinity for HLA class I differentiate into CD8+, those with intermedate affinity for HLA class II differentiate into CD4+ cells

Question 55

What are CD8+ T cells also called? What are their functions?

Answer 55

Cytotoxic cells 1. Recognise peptides derved from INTRACELLULAR proteins in association with HLA class I 2. Kill cells via perforin, granzymes and expression of Fas ligand 3. Secrete cytokines e.g. IFNgamma, TNFalpha *Defence against tumours and viral infections*

Question 56

What are 3 functions of CD4+ 'helper' T cells?

Answer 56

1. Recognise peptides derived from extracellular proteins presented on HLA class II molecules (HLA-DR, HLA-DP, HLA-DQ) 2. Provide help for development of full B cell responses 3. Povide help for development of some CD8+ T cell responses (2 and 3 are immunorregulatory functions via cell:cell or cytokine interactions)

Question 57

What are polarising factors?

Answer 57

* CD4+ T cells can differentiate into further subtypes of CD4+ T cell * Certain polarising factors will lead to the development of subtypes of T cell with different effector profiles

Question 58

T Cell Deficiency: There can be an issue with bone marrow = production of lympocytes (SCID), or issues with T-cell maturation in the thymus or issues with failure of signalling, cytokine production and effector functions of T-cells (e.g IFNγ or IFNγR deficiency, IL-12 or IL-12R deficiency) list 2 issues with T-cell maturation?

Answer 58

1. failure of thymic development: DiGeorge syndrome 2. failure of expression of HLA molecules: Bare lymphocyte syndromes

Question 59

What DiGeorge syndrome - mutations responsble, clinical features and lymphocyte levels

Answer 59

22q11.2 delection syndrome - most cases are **sporadic** rather than inherited = Developmental defect of pharyngeal pouch = other abnormalities from the forehead to the chest Clinical features: Remember CATCH-22: * Cardiac abnormalities (especially tetralogy of Fallot) * Abnormal facies (high forehead, low set ears, small jaw) * Thymic aplasia (T cell lymphopenia) * Cleft palate * Hypocalcaemia/hypoparathyroidism * 22 – chromosome

Question 60

What are the lymphocyte levels in 22q11.2 deletion syndrome and do you need to have a transplant?

Answer 60

**Normal numbers B cells and reduced numbers T cells** **Reduced number of T cells - lack of thymus =** do not select T cells well- do get T cells but not as many - **-** *homeostatic proliferation with age so immune function usually only mildly impaired and improves with age* * Over time, they tend to proliferate in a homeostatic way driven by cytokines * Tend to fill up the T cell compartment with time * So immune function is only mildly impaired and gets better with age * Thus, do NOT need to do a transplant usually

Question 61

Issues with T-cell maturation: what is the syndrome caused by failure of expression of HLA molecules and explain it?

Answer 61

Note: (BLS type 1 also exists due to failure of expression of HLA class I) BLS II - HLA Class 2 more common

Question 62

Which regulatory proteins contain mutations in bare lymphocyte syndrome type 2? What phenotype does this cause?

Answer 62

**_Regulatory factor X_** or **_Class II transactivator_** --\> absence of MHC II molecules _Phenotype:_ * Deficiency of CD4+ cells * Normal number of CD8+ cells * Low or normal IgG and IgA due to lack of CD4+ T cell help in germinal centres *BLS type 1 is where MHC I is absent.*

Question 63

Why are IgA and IgG levels low in BLS type 2?

Answer 63

No CD4+ T cells which help with isotype switching to change IgM into IgA and IgG in the germinal centres

Question 64

What are the clinical features of BLS?

Answer 64

* Unwell by 3 months * Infections of all types * Failure to thrive * FH of early infant death

Question 65

Name 4 types of defects of T cell effector function.

Answer 65

Defects in: * cytokine production - IFN * cytokine receptors - IL-12 * cytotoxicity * T-B cell communication (CD40 ligand deficiency)

Question 66

What are the clinical features of T lymphocyte deficiency?

Answer 66

* Viral infections e.g. CMV * Fungal infections e.g. pneumocytsic, cryptosporidium * Some bacterial infections esp intracellular organisms e.g. TB, salmonella * Early malignancy

Question 67

What are the main investigations for T cell deficiencies?

Answer 67

* **Total white cell count and differential** - remember that lymphocyte counts are normally much higher in children than in adults * **Lymphocyte subsets** - quantify CD8 T cells, CD4 T cells as well as B cells and NK cells * **Immunoglobulins** - if CD4 T cell deficient * **Functional tests of T cell activation and proliferation** - useful if signalling or activation defects are suspected * **HIV test**

Question 68

Which markers can be used for B and T cell sorting FACS?

Answer 68

T cells - CD8 and CD4 marker OR CD3 (both) B cells - CD20

Question 69

Fill in the blanks.

Answer 69

Question 70

What is the management of T cell immunodeficiency?

Answer 70

* **Aggressive prophylaxis/treatment of infection** * **Haematopoieitic stem cell transplantation** - to replace abnormal populations in SCID; to replace abnormal cells -class II deficient APCs in BLS * **Enzyme replacement therapy** - PEG-ADA for ADA SCID * **Gene therapy** - stem cells treated ex-vivo with viral vectors containing missing components, transduced cells have survival advantage in vivo * **Thymic transplantation** - to promote T cell differentiation in Di George syndrome - cultured donor thymic tissue transplanted to quadriceps muscle

Question 71

Match these up.

Answer 71

1, 2 2, 4 3, 3 4, 1

Question 72

Describe B cell development briefly. In what form do B cells leave the BM?

Answer 72

IgM B cells

Question 73

How is central tolerance by B cells maintained?

Answer 73

No recognition of seld in BM means that B cells survive and leave BM (those that recognise self are negatively selected to avoid autoreactivity) NB: NO INTERMEDIATES

Question 74

What happens to B cell upon antigen encounter?

Answer 74

1. _Early IgM response_ - T cell independent whereby IgM memory cells and Ab secreting plasma cells are found 2. _Germinal centre reaction in lymph node with CD4+ T cell help_ 1. Dendritic cells prime CD4+ cells 2. CD4+ cells help B cells differentate (via CD40L:CD40 interaction) 3. B cell proliferation, **somatic hypermutation**, i**sotype switching** to IgG, A, E --\> **high affinity memory cell and plasma cell production.**

Question 75

What are Igs made up of?

Answer 75

2 heavy and 2 light chains 5 types - IgA, E, G, D, M (and subclasses of IgG and IgA) Antigen is recognised by **antigen binding region Fab (V)** Effector function determined by **constant region Fc (stem)**

Question 76

Name 3 immune components/cells which antibodies interact with.

Answer 76

Complement Phagocytes NK cells

Question 77

What is the name for condition in which there is failure for preB-cells to differentiate into B cells?

Answer 77

Bruton's X-linked hypogammaglobulinaemia

Question 78

What is the mutation in Bruton's X linked agammaglobulinaemia? What is the phenotype and when does it manifest?

Answer 78

Abnormal B cell tyrosine kinase (BTK) gene Absence of MATURE B cells and no IgG after 3 months (ONLY IN BOYS)

Question 79

What are the clinical features of X linked agammaglobulinaemia?

Answer 79

* Boys (present in first few years of life) * Recurrent bacterial infection - otitis media, sinusitis, pneumonia, osteomyelitis, septic arthritis, gastroenteritis * Viral, fungal, parasitis infections - enteroviruses, pneumocystis * Failure to thrive

Question 80

Name a B cell maturation defect. What is the inheritance pattern?

Answer 80

Hyper IgM syndrome (X linked recessive)

Question 81

What is the mutation in Hyper IgM syndrome? Where is this expressed: B or T cells?

Answer 81

**Mutation in CD40 ligand gene (CD40L, CD154) so that CD4+ T cells cannot offer help to B cells in germinal centres** Encoded on Xq26, member of TNF-R family DEFECT IS ON THE T CELL

Question 82

What are the antibody levels in hyper IgM syndrome?

Answer 82

Undetectable IgA, IgE and IgG Elevated serum IgM (no germinal centre development and no isotype switching)

Question 83

How does hyper IgM syndrome present clinically?

Answer 83

In boys as with Bruton's Recurrent bacterial infections BUT abnormality in T cell also predisposes to PCP, AI disease and malignancy Failure to thrive

Question 84

What is the phenotype of common variable immune deficiency? (CVID) What is the pathophysiology?

Answer 84

Defined by (1) **low IgA, IgG and IgE** (--\> recurrent bacterial infections,) (2) **poor response to immunisation**, (3) absence of o**ther immunodeficiency** COMMON and can present in childhood or adulthood with heterogenous genetic defects Due to failure of full differentation/function of B lymphocytes

Question 85

What are the clinical features of CVID?

Answer 85

Adults OR children affected **Recurrent bacterial infections** - pneumonia, persistent sinusitis, gastroenteritis, often with severe end-organ damage **Pulmonary disease** - interstitial lung disease, granulomatous interstitial lung disease (also LN, spleen), obstructive airways disease **Gastrointestinal disease** - IBD like disease, sprue like illness, bacterial overgrowth **Autoimmune disease** - AIHA or thrombocytopenia, RhA, pernicious anaemia, thyroiditis, vitiligo **Malignancy** - NHL

Question 86

How common is IgA deficiency? What is the cause?

Answer 86

Prevalence high with 1:600 affected but only 1/3 are symptomatic (recurrent resp infections) Genetic component, cause yet unknown

Question 87

Question 88

Which bacterial infections, toxins and viral infections are most common in antibody deficiencies/CD4 T cell deficiency?

Answer 88

Bacterial - staphylococcal, streptococcal Toxins - tetanus, diptheria Viral - enterovirus

Question 89

Which investigations are used for B cell deficiencies?

Answer 89

* **Total WCC and differential** - lymphocyte counts in general are much higher in children than in adults * **Lymphocyte subset**s - to quantify B cells, CD4, CD8 and NK cells * **Serum Igs and protein electrophoresis** - production of IgG is a surrogate marker for CD4 T helper cell function * **Functinal tests of B cell function** - measure specific Ab reponses to known pathogens/immunisations e.g. tetanus, HIB, S pneumonia; if low then immunise and repeat measurement in 6-8 weeks.

Question 90

What is shown here? What condition could this be?

Answer 90

Gamma band shows antibodies - this one above shows absence so this could be Bruton's

Question 91

Fill in the blanks.

Answer 91

Question 92

How do you manage B cell immunodeficiency? Is HSCT curative?

Answer 92

1. **Aggressive prophylaxis / treatment of infection** 2. **Immunoglobulin replacement if required** - derived from pooled plasma from thousands of donors; contains IgG antibodies to a wide variety of common organisms, aim of maintaining trough IgG levels within the normal range; treatment is life-long 3. **Immunisation** - for selective IgA deficiency, not otherwise effective because of defect in IgG antibody production HSCT not required as you can keep replacing antibodies instead.

Question 93

Match these up.

Answer 93

1, 2 2, 4 3, 3 4, 1

Question 94

QUESTION 1: Young child with recurrent infections. High neutrophil count on FBC but no abscess formation (choose correct answer + explain it) Chronic granulomatous disease Classic NK cell deficiency IFNγ receptor deficiency Kostmann syndrome Leukocyte adhesion deficiency

Answer 94

Defect of Phagocyte Migration- Leukocyte Adhesion Deficiency This is due to a deficiency of CD18 (β2 integrin subunit) CD18 binds to CD11a forming a molecule called LFA-1, which is expressed on neutrophils. This binds to the ligand (ICAM-1) on endothelial cells and so regulates neutrophil adhesion/ transmigration In LAD, the neutrophils lack these adhesion molecules and FAIL to exit from the blood stream Very high neutrophil counts in the blood ABSENCE of pus formation

Question 95

QUESTION 2: Child with recurrent infections with hepatosplenomegaly and abnormal dihydrorhodamine test (does not fluoresce) NOTE: dihydrorhodamine test- take phagocytes and neutrophils and activate them to induce an oxidative burst. Then put in dihydrorhodamine which fluoresces as neutrophils produce ROS as result of this oxidative burst [i.e +ve if oxidative killing present] Chronic granulomatous disease Classic NK cell deficiency IFNγ receptor deficiency Kostmann syndrome Leukocyte adhesion deficiency

Answer 95

Failure of Phagocytic Oxidative Killing mechanisms- Chronic granulomatous disease Absent respiratory burst Normally when neutrophils are activated, there is a respiratory burst and H2O2 is produced Deficiency of one of the components of NADPH oxidase leads to an inability to generate oxygen free radicals, resulting in impaired killing Phagocytes will try and phagocytose the pathogen but will not be successful Excessive inflammation Persistent neutrophil/ macrophage accumulation Failure to degrade antigens Granuloma formation Lymphadenopathy and hepatosplenomegaly