W12 Adaptive immune system - focus on B cells

Question 1

Innate immune system

Answer 1

Rapid response
Non-specific (generic anti-bacterial or anti-viral mechanisms)
Most often fails to completely eliminate the infection

Question 2

Adaptive immune system

Answer 2

Delayed response
Highly specific
Usually eliminates infection
Memory
Long term immunity, but specific to that particular pathogen

Question 3

Adaptive Immunity

Answer 3

Humoral immunity
Mediated by B-lymphocytes

Cellular immunity
Mediated by CD8+ cytotoxic T- lymphocytes

Both branches regulated by CD4+ helper T-lymphocytes
(T-helper cells)

Question 4

Humoral immunity

Answer 4

Humor = fluid

Following an infection
Plasma contains substances- “antibody (Ab)” -which neutralise that specific infectious agent
Demonstrate in vitro
Or in vivo, e.g. treatment of rabies by infusion of antibody
“adoptive immunotherapy”

Question 5

What is antibody?

Answer 5

Protein- “immunoglobulin (Ig)”
Migrates in the γ-globulin fraction on serum electrophoresis
Each antibody binds to a specific antigen (most often a protein) on the infectious agent
But plasma contains many different Abs
Note how diffuse the γ-globulin band is.

Question 6

Antibody - structure

Answer 6

Immunoglobulin protein
Y-shaped
Tetrameric 
2 identical heavy chains
2 identical light chains
Held together by non-covalent interactions and by –S-S- crosslinks between cysteine a.a. residues

Question 7

Light Chains

Answer 7

There are two types of light chain
Kappa (κ) and lambda (λ)
But any B-cell will only make one type
Any Ig molecule will contain either kappa or lambda, never both

This phenomenon is called “light chain restriction”

Question 8

Each chain has a variable region

Answer 8

Amino acid sequence varies from one Ig molecule to another
Binds antigen

And a constant region
Responsible for effector functions
E.g. activating complement, binding to phagocytes

Question 9

antibody (Ab) - structure

Answer 9

Each Ig molecule has two antigen binding sites

And a flexible hinge region

Ig is a glycoprotein
Carbohydrate added in the Golgi

Question 10

Fab and Fc Antibody fragments

Answer 10

Ig treated briefly with protease. Cuts molecule at hinge region

Fab- fraction
Antigen binding

Fc- fraction
crystallisable

Question 11

How does antibody fight infection?

Answer 11

By coating and neutralising a pathogen
E.g. if a virus is coated with Ab it cannot bind to its receptors on the cell surface

By activating complement
Which can then blow holes in a bacterial cell membrane

By opsinisation
Phagocytes have Fc receptors on their cell membrane
Bind to pathogens coated with Ab, and phagocytose them

Question 12

How does an Ab bind to antigen?

Answer 12

Non-covalent interactions
- Electrostatic, hydrophobic, van der Waals forces, hydrogen bonds

• Depends on the antibody binding site being exactly complementary, sterically and chemically, with a site on the surface of the antigen
• The binding site on the Ag for one specific Ab is called an epitope

Question 13

Different types of B cells

Answer 13

The body generates over 100,000,000 different B-cells each making a different “random” Ig
Each B-cell only makes one specific Ig
These naïve B-cells sit around in lymph nodes doing not very much

Question 14

During an infection - B cells

Answer 14

During an infection, a small number of B-cells will, by chance, be making an Ig that binds one of the foreign antigens
These B-cells are activated and begin to multiply- “clonal selection”

Question 15

Clone

Answer 15

a group of cells (or organisms) that are genetically identical

Descendants of the original activated B-cell make the same Ig
Therefore they are a clone

Question 16

Lymphocyte Development in the Bone Marrow

Answer 16

HSC into CLP + CMP (Neutrophils, red cells, platelets etc)

CLP into Pre-T (further development in thymus) + Pre-B

Pre-B into imm-B

HSC: haematopoietic stem cell
CMP: common myeloid progenitor
CLP-:common lymphoid progenitor
Pre-T: T-cell precursors

Question 17

Primary and Secondary Lymphoid Organs

Answer 17

HSC into Pre-B into Imm B-cells

Imm B-cells into follicles containing resting B-cells (secondary lymphoid organs, lymph nodes, spleen, gut etc)

Question 18

B-cell activation

Answer 18

Functional Ig is first expressed as IgM on the cell surface
(sIgM)

This acts as a “B-cell receptor” in a similar way to a growth factor receptor. The IgM does not have intrinsic tyrosine kinase activity, but associates with other tyrosine kinases

Binding of antigen to IgM activates the tyrosine kinases and their signal transduction pathways

Question 19

B-cell activation requires

Answer 19

Antigen binding to the B-cell receptor (sIgM), resulting in stimulation of signal transduction pathways
Co-stimulation by T-cells

The activated B-cell begins to secrete soluble IgM

Question 20

Activated B cells

Answer 20

B cells activated
Multiply rapidly
Differentiate to become Ig secreting cells

First make IgM

Then undergo class switching to make Igs with
the same Ag specificity
but different heavy chain constant regions
E.g. IgG, IgA etc

Ig secreting cells e.g. plasma cells
Memory B- cells

Question 21

Memory B-cells

Answer 21

Memory B-cells allow a very rapid response to a second exposure
Immediate production of IgG rather than IgM

Question 22

Natural immune responses are polyclonal

Answer 22

More than one clone of B-cells is generated
More than one Ig is synthesised

Because
Multiple antigens on organism
Multiple epitopes on each antigen
More than one Ig may recognise the same epitope

Question 23

Class (or isotype) Switching

Answer 23

Once a B-cell starts making an Ig which binds a specific Ag:
It can switch to make Igs with the same Ag binding site
But different constant regions
To carry out different functions in different parts of the body

Question 24

Classes (isotypes) of Ig

Answer 24

The body can make different classes of Ig
IgG, IgM, IgA, IgD, IgE
differ slightly in heavy chain constant region amino acid sequence
Have different functions

note: 
there are actually 4 types of IgG (subclass IgG1 – IgG4)
And 2 types of IgA (subclass IgA1  and IgA2)

Question 25

Heavy chain isotype

Answer 25

gamma = IgG

miu = IgM

alpha = IgA

delta = IgD

epsilon = IgE

Question 26

IgM

Answer 26

Always the first class of Ig made by B-cells during the primary response
First made as a membrane bound protein on B-cell surface
Activates B-cell by signal transduction

Later made in secreted form
Activates complement
Acts as opsonin

Question 27

IgM - presence of specific IgM antibodies

Answer 27

Presence of specific IgM antibodies to an antigen indicates a recent primary response to that antigen
Implies a current primary infection

Presence of IgG antibodies may be due to past exposure to antigen

Question 28

IgM Structure

Answer 28

Membrane bound IgM is formed of a single Ig tetramer

In secreted IgM five molecules of the basic Ig tetramer polymerise to form a pentamer

Question 29

IgG

Answer 29

Major class of Ig in the circulation
Very good at activating complement system
Good as an opsonin

Formed of a single Ig tetramer

Question 30

IgA

Answer 30

Most abundant class in external secretions
Milk, sweat, tears, gut secretions
Protects mucosal surfaces
Does not activate complement
Does bind Fc receptors triggering
Phagocytosis
Inflammatory reactions

Question 31

IgA structure

Answer 31

In serum, occurs as a single Ig molecule

In secretions, most IgA is present as a dimer of two whole Ig molecules (+ accessory proteins)

Question 32

IgE

Answer 32

Physiological role in protection against parasitic worms
Binds to Fc receptors on mast cells and basophils
Triggers release of histamine

BUT also involved in allergies!
IgE produced in response to allergens (pollen, peanuts etc)
Release of histamine causes symptoms of allergies
Over response can cause anaphylactic shock

Question 33

IgD

Answer 33

Extremely low concentration in circulation
Also found on B-cell membrane
Role is unknown