Humoral Immunity Flashcards
What is humoral immunity?
Immunity that is mediated by secreted antibodies and peptides against extracellular microbes and toxins (antibody, complement)
Describe the 2 ways in which B cells can be activated.
- Antibodies are secreted by _____ cells
- Perform effector functions _____ (relative location)
- Neutralize and eliminate infectious _____ and microbial _____
- plasma
- distally
- microbes, toxins
What is the difference between the primary and secondary Ab responses?
Primary response = when antigen is first encountered, usually slow response time and takes about 1 week
Different Isotypes allow the antibodies to have different ____ functions.
Effector
The effector functions are triggered by the binding of____ to the variable regions.
antigens
Effector mechanisms not active when not bound to ____.
antigen
What are 4 major effector functions of antibodies?
Activate the complement system
Neutralize mircobes and toxins
Opsonization and phagocytosis
Ab dependent cellular toxicity
Describe how an antibody would neutralize a microbe or toxin.
- The antibody can bind to microbes to block their entry into tissues through the epithelium or block their ability to bind to receptors on cells that would mediate their entry into the cell.
- The antibody can bind to the toxin and prevent the toxin from binding to the cellular receptor to produce a response.
What is opsonization? What is an opsonin?
Opsonization = process of coating particles for phagocytosis
Opsonin = molecule that coats microbes to enchance phagocytosis
Describe the process of opsonization and phagocytosis.
IgG (opsonin) binds to the microbe and coats the microbe with Ab (opsonization). Phagocytic cells express Fc_gamm_R1, which recognizes the constant region of IgG. The receptor binds IgG which activates the phagocyte and results in internalization of the microbe and Ab and subsequent degradation of the Ab and microbe inside the phagocyte.
What antibody is important for opsonization of microbes?
IgG
What is antibody dependent cellular toxicity?
IgG binds to antigen on the surface of a cell (could be microbe/pathogen, could be self) –> NK cells expressing Fc_gamma_RIII receptor recognize the cell and receptor binds to constant domain of IgG –> activates NK cell to release granules which kill the Ab coated cell.
How do antibodies play a role in killing parasites (helminths)?
Helminths are too large to be phagocytized. Instead, circulating IgE binds to surface antigen on the helminths and coats them in Ab. The eosinophil expresses receptors on surface for constant region of IgE. When eosinophil binds to IgE, it is activated (with help of Th2 cell and IL-5) and it degranulates releasing major basic protein (toxic, cationic protein) which kills the helminth.
How do antibodies activate the complement system?
The classical pathway of the complement system requires antibody to become activated.
What are the 3 major outcomes of the complement system?
Opsonization and phagocytosis
Cytolysis (MAC)
Inflammation
What are the 3 goals of vaccination?
Safe
Produce life-long immunity
Produce immunity in large % population
- What is a life-attenuated vaccine?
- What are the pros?
- What are the cons?
- A vaccine that consists of an intact microbe that is altered in a way that it can no longer cause disease
- Produces innate and apative immune response, activates humoral and cellular immunity components, produces life-long immunity
- In rare cases, viruses that are delivered this way can become reactivated and cause disease; these are not safe for those who are immunocompromised
- What is a purified antigen vaccination?
- What additional compound are these delivered with?
- What are the pros?
- What are the cons?
- Antigens purified from microbes OR inactivated toxins
- An adjuvant that is needed to stimulate the innate immunity
- Elicits both innate and adaptive immune responses and does not have a risk of reactivation
- The response is generally weaker and boosters may be required
What is serum protein electrophoresis?
Essentailly just gel electrophoresis that separates proteins in the serum used in tandem with a density meter that looks at the density of various bands on the gel. Abs are found in the gamma peak / band.
What are monoclonal antibodies?
Identical antibodies that have the same specificity
Describe how monoclonal Abs are made.
A sample of B cells from an organism’s spleen are obtained and fused with a mutant myeloma lineage cell. These cells are then grown in selective media (HAT) that only allows for hybrid cells to grow. The hybrid cells are then selected for by separating into different aliquots and testing for binding to antigen. Positive clones that were selected are separated and cloned to produce a mass of cells that only secrete one Ab.
Describe the process of Flow cytometry & Fluorescence-activated cell sorting (FACS)
A mixed population of cells are tagged with specific monoclonal Abs that are conjugated to a fluorophore. The fluid is passed through a nozzle that releases the fluid in single droplets that tend to contain only one cell at a time. The fluid passes past a laser, that produces light that passes through a filter to a detector that detects the color of the fluorophore that is attached and thus the Ab that is attached. This information is transmitted to the computer, which then changes the charge on sorting plates so that when the droplet passes through the plates it can be directed via a change in current to a specific collection vessel that will only contain cells with the same Abs bound.
What does flow cytometery tell you?
Size and granularity of cells
Whether the cell is (+) or (-) for an Ab and by proxy surface Ag
What are the 3 branches of the complement system?
Alternative pathway
Classical pathway
Lectin pathway
Complement protein C2 is unique in that it’s cleavage product 2a is _______ than cleavage product 2b.
Larger
Alternative pathway
- What initiates this pathway?
- Is Ab required?
- Spontaneous degradation of C3 into C3a and C3b
- No
*
What is C3 tickover?
Spontanous cleavage of C3 into C3a and C3b, occurs at rate of approx 1-2% of total C3 / hour in plasma
- Once C3 has been spontaneously cleaved into C3a and C3b, what is revealed on the protein?
- What are the 2 outcomes for that C3b?
- Cleavage reveals reactive thioester groups
- C3b can be rapidly hydrolyzed by water or thioester can bind to microbe surface (via binding to amino / OH groups on microbial proteins / polysaccharides)
Cleavage of C3 into C3a and C3b reveals the binding site for what other complement protein?
B
What molecule stabilizes C3b binding to microbe?
Properdin
What factor is necessary to form the C3 convertase?
Factor D –> Cleaves B into Bb and Ba
The C5 convertase consists of what complement proteins?
C3b – Bb – C3b
What substances produced in the alternative pathway stimualte inflammation?
C3a and C5a
What is the fate of C3b after it has been cleaved by the C3 convertase?
It can bind to the C3 convertase to form a C5 convertase or it can bind to the microbe to amplify the complement cascade.
What is the end result of the complement cascade?
C5b – C6 (combines in plasma and migrates to membrane where C6 imbeds in membrane of microbe)
C5b – C6 – C7 (membrane imbedded)
C5b – C6 – C7 – C8 (membrane imbedded)
C5b – C6 – C7 – C8 – C9 (membrane imbedded)
C5b – C6 – C7 – C8 – Poly C9 => Cytolysis via polyC9 pore
What is needed to stimulate the classical pathway?
IgM or IgG binding to microbe first
Describe the classical pathway.
2 heads of C1q bind to constant region of Ab –> conformational change in C1q –> stimulates conformational change in C1r2 and C1s2 –> C1r activates C1s –> C1s is serine protease –> cleaves C4 to C4a (inflammation) + C4b –> C4b binds to microbe surface –> binds to circulating C2 –> C1s cleaves C2 into C2a (remains bound) and C2b –> C4b – C2a complex is C3 convertase –> C3a (inflammation) + C3b –> C3b can bind to microbe or to C4a – C2b complex to form C5 convertase –> convert C5 to C5a (inflammation) and C5b –> triggers cytolysis
The lectin complement pathway is most similar to which other part of the complement system?
Classical pathway
What do lectins in the lectin pathway bind to ?
PAMPS
How does the complement system lead to opsonization and phagocytosis?
Microbes become coated with C3b or C4b which are recognized by receptors on phagocytes and recognition leads to phagocytosis
How does the complement system lead to inflammation?
C3a and C5a bind to receptors on mast cells –> degranulation –> release histamine –> vasodilation –> increased blood flow, increased vascular permeability, increased fluid and lymphatic drainage
What is cR1 (aka CD35)?
Where is it expressed?
What is its function?
The receptor for C3b and C4b
Expressed on surface of phagocytes
Promote phagocytosis
What is cR2 (aka CD21)?
Where is it expressed?
What is its function?
C3b can be further cleaved to smaller products that bind to microbes. This receptor binds to cleavage products of C3b on microbes.
B cells, Dendritic cells
Signals B cells to enhance its response to the antigen
Why is it necessary to regulate the complement system?
- Spontaneous activation damages host
- Activated byproducts can diffuse and cause host cell injury (inflammation)
C1 Inhibitor (C1 INH)
- What type of molecule is this?
- What does it inhibit and how?
- SERPIN = soluble serine protease inhibitor
- C1r2s2 and MASP proteins –> C1 INH binds to these proteins and causes them to dissociate from either C1q or lectin
Decay Accelerating Factor (DAF)
- What part of the complement system does this inhibit?
- Components of the C3 convertase by blocking progression of the cascade
Factor I
- What type of molecule is this?
- What part of the complement does it inhibit and how?
- What does it generate in its action?
- Serine protease
- Degrades C3b and C4b
- Produces byproducts of C3b degradation that bind to microbes and then bind to cR2 on host cells to sensitize b cells to antigen
What are 2 ways in which MAC can be inhibited?
- CD59 - inserts itself into assembling MACs and prevents addition of further C9s
- S protein - binds to C5b, C6 and C7 to prevent insertion of C9
What condition would arise if a person were deficient in either DAF (CD55) or CD59?
They would have RBC lysis because they would be unable to properly regulate complement activity in erythrocytes, which would lead to recurrent bouts of hemolysis
What would happen if a person were deficient in C3?
What would happen if they were deficient in any of C5 - C9?
What does this tell you about the relative functions of the complement system?
- Deficiency in C3 leads to increased risk for severe infections and can be fatal
- Deficiency in C5 - C9 often does not result in any symptoms, maybe only slight increased risk for certain infections
- Suggests that the complement system’s ability to produce inflammation is >>> important than its ability to perform cytolysis
C1 INH Deficiency
- What builds up in this condition?
- How do patients present / why?
- Fragments of C3, C4 and C2
- With severe inflammation (edema, pain, diarrhea, life threatening airway obstruction) b/c C3a, C4a and C2b all lead to inflammation
