Immuno Lec 8 - B cells and antibodies

Question 1

why do B cells have their name

Answer 1

because they mature in the bursa of fabricius in chickens

Question 2

development of B cells

Answer 2

B cells arise from bone marrow precursors. B cells develop in the bursa (in birds), Peyer’s patches (lining the small intestine; in sheep and cattle), or bone marrow (humans, cats, dogs

Question 3

stages of B cell development

Answer 3

-B cells originate in the bone marrow and proceed through a series of differentiation stages before becoming able to respond to antigens. When B cells respond to epitopes on antigens, they proliferate and differentiate into plasma cells

Question 4

what are plasma cells

Answer 4

B cells that actively secrete antibodies
are known as plasma cells

Question 5

immunoglobulin vs antibody

Answer 5

• An immunoglobulin is of unknown
  specificity
• An antibody is an immunoglobulin
  with a known specificity
• Immunoglobulins/antibodies are
  secreted versions of the B cell receptors

Question 6

the difference between antigens recognized by T vs B cells/antibodies

Answer 6

• T cells recognize linear epitopes
  (peptide fragments) derived from
  the processing of protein antigens
  and they need to presented in the
  context of MHC molecules
• B cells can recognize proteins,
  polysaccharides and lipids in their
  native 3‐D form (i.e., unprocessed)

Question 7

immunoglobulin structure

Answer 7

Immunoglobulins consist of two heavy and two light chains bound together by disulfide bonds

Question 8

antigen binding region of an antibody

Answer 8

The variable regions of the light and heavy chains of an immunoglobulin molecule are divided into three highly variable complementarity‐ determining regions separated by relatively constant framework regions.

-The way in which the complementarity‐ determining regions are folded to form the antigen binding site on an immunoglobulin molecule. A similar folding occurs in the peptide chains of the TCR

Question 9

antibodies; generation of diversity (7 terms)

Answer 9

-Gene rearrangement: V (variable)‐J (joining) and V‐D (diversity)‐J gene
recombination (similar to T cell receptors)

-Base deletion: an endonuclease can randomly remove a base from the end of one of the above genes before they are spliced

-Base insertion: additional nucleotides can be inserted at gene splice sites by terminal deoxynucleotidyltransferase

-Somatic mutation: random mutations introduced when cytidine deaminase converts cytidines to uracils (not normally present in DNA) – these uracils get “repaired” by swapping in a different nucleotide

-Combinatorial association: heavy chains can associate with different light chains (e.g., κ versus λ)

-Gene conversion: cytidine deaminase converts cytidines to uracils – these get “repaired” by insertion of short segments of DNA derived from other antibody genes or pseudogenes (non‐functional on their own)

-Receptor editing: antibodies using κ light chains can either switch to other κ V genes or to a λ light chain

-The number of different antigen‐binding specificities generated is about 1.8 × 10 16

Question 10

affinity maturation and what it means

Answer 10

• The mechanisms that generate antibody diversity result in a process known as affinity maturation
• This means the binding affinity of an
  antibody can be enhanced as an immune response progresses
• Note that these processes might also result in lower affinity or even dysfunctional antibodies that cause deletion of the B cell

Question 11

five immunogloblin isotypes in mammals

Answer 11

-IgG, IgA, IgM, IgE, IgD

-four IgG (1, 2, 3, 4) subclasses
-two IgA subclasses (1, 2)
-two L chain types (k and lambda)

Question 12

what immunoglobulin associated with allergies, which is best at triggering membrane attack complex

which has longest half life

Answer 12

-IgE is present to get rid of parasites which we want. an unfortunate side effect is allergies

-IgM is best at triggering membrane attack complex

IgG has longest half life (26d)

Question 13

Ig subclasses and immune response bias

Answer 13

-a response against an intracellular pathogen. IgM:IgG2a = 18:1… Total Ig = ~288ng/mL

-a response against an extracellular pathogen. IgM:IgG2a = 2513:1… Total Ig = ~120,853ng/mL

so very different orders of magnitude

Question 14

role of mammalian immunoglobulin isotypes

Answer 14

• IgG is the predominant immunoglobulin in serum and is responsible for systemic defense
• IgM is a very large immunoglobulin mainly produced during a primary immune response
• IgA is the immunoglobulin responsible for the defense of mucosal surfaces (e.g., intestinal and respiratory tracts)
• IgE is found in very small quantities in serum and is responsible for immunity to parasitic worms and for allergies
• IgD is found on the surface of immature lymphocytes. Its function is unknown

Question 15

affinity vs avidity

Answer 15

• Affinity: The strength of the binding between a single binding site of a molecule (e.g., an antibody) and a ligand (e.g., an antigen).
• Avidity: The overall strength of interaction between two molecules, such as an antibody and an antigen. The avidity depends on both the affinity and the valency of interactions. Therefore, the avidity of a pentameric IgM antibody, with 10 antigen‐binding sites, for a multivalent antigen may be much greater than the avidity of a dimeric IgG molecule for the same antigen

Question 16

T dependent vs T independent antibody responses

Answer 16

• Based on the requirement for T cell help (i.e., cytokines from CD4+ T cells)
• T‐dependent: responses to protein antigens (without CD4+ T cell activation, these Ab responses are weak)
• T‐independent: Polysaccharides, lipids, and other non‐protein antigens stimulate antibody production without the involvement of helper T cells (which only respond to protein antigens)

Question 17

typical T dependent Ab response

Answer 17

-The sequence of events that must occur for a B cell to respond to an
antigen. Not only must the B cell be stimulated by an antigen, but it must also receive co‐stimulation from helper T cells and their cytokines

Question 18

primary vs secondary Ab responses

Answer 18

PRIMARY
-lag after immunization: 5-10days
-peak response: smaller
-antibody type: usually IgM>IgG
-antibody affinity: lower average affinity, more variable

SECONDARY
-lag after immunization: usually 1-3d
-peak response: larger
-antibody type: relative increase in IgG and under certain situations, in IgA or IgE (heavy chain isotype switching)
-antibody affinity: higher average affinity (affinity maturation)

Question 19

time course of a B cell response

Answer 19

-The cellular events that accompany a B cell response. Note how some IgG is made in the primary immune response, whereas a small amount of IgM is made in a secondary immune response. Isotype switching and affinity maturation occurs as the response progresses

Question 20

effector functions of antibodies

Answer 20

-aggulutination = antigenic particle + specific Ab results in aggregation of particles

-precipitation = soluble Ag + specific Ab results in lattice formation and precipitation

-C activation = Ag in solution or on particle + specific Ab results in activation of C

-Cytolysis = cell + anti-cell Ab + C may result in lysis of the cell

-Opsonization = Antigenic particle + Ab + C enhances phagocytosis by Mo, M) and PMNs

-Neutralization = toxins, viruses, enzymes, etc + specific Abs may result in their activation

-antibody dependent NK cell mediated cytotoxicity –> The killing of antibody‐coated target cells by natural killer cells with surface Fc receptors.

Question 21

B cells can function as APCs

Answer 21

-B cells can function as antigen‐
presenting cells for the activation of primed (i.e., previously‐activated/memory) CD4+ helper T cells. They do this by sending antigens captured by the B cell receptor into the exogenous antigen processing pathway and then presenting epitopes in the context of MHC class II (signal 1) in conjunction
with the expression of co‐ stimulatory molecules (signal 2)

(signal 2 would also need to be delivered to activate the cell)

Question 22

clarification about antigen presentation

Answer 22

• All cells (except red blood cells/erythrocytes) can present antigen‐derived epitopes in the context of major histocompatibility complex (MHC) class I molecules (to potentially activate CD8+ cytotoxic T cells).
• Only antigen‐presenting cells (APCs) express MHC class II (to recruit CD4+ T cell help).
• Only dendritic cells and some macrophages are capable of antigen cross‐presentation

Question 23

cancerous B cells

Answer 23

If B cells become transformed, they cause leukemias:
*B lymphoblastic (immature B cell)
*B lymphocytic (mature B cell)
*Multiple myeloma (plasma cell)
…or a lymphoma (solid tumour)

Question 24

cancerous plasma cells as a useful tool

Answer 24

• If a myeloma cell (i.e., cancerous plasma cells) is fused with normal plasma cells a new cell line can be generated with the following characteristics:
  1. Massive antibody‐producing capability of the myeloma cell

2. Specificity of the antibody form the normal cell
   * This is how monoclonal antibodies are made
   * These can be used for therapeutic, diagnostic and research purposes