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IMM250H1 Study Guide - Final Guide: Immunoglobulin Heavy Chain, B-Cell Receptor, Antigen-Presenting Cell

Course Code
Dana Philpott
Study Guide

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Lecture 6  Lymphocyte Development
Cells of the Adaptive Immune System
oLymphoid stem cell
B-cell progenitor
oPlasma cell
oMemory cell
T-cell progenitor
oCD4+ Th cell
oCD8+ Th cell
oB Lymphocyte
Creates Antibodies –neutralize pathogens/toxins
Antibodies look for infected cells and scope out antigens
via antibody molecules
oAntigens serve as a target of the adaptive
immune response, or the antibodies
oT Lymphocyte
T cell receptors recognizes short peptide fragments of pathogen
protein antigens, bound to MHC molecules on other cell
don’t recognize and bind antigens directly
MHC molecules are involved in presentation of peptide
antigens to T cells
T cells then secrete cytokines: molecules that signal
production of molecular killers
Memory for the event is created
oLymphocytes mediate adaptive responses by recognizing antigens
PAMPs are simple and pathogen-associated, and recognized by
innate leukocytes
Antigens are complex, and pathogen-specific
oFound on bacteria, viruses, Helminth/parasites
Types of antigens:
“self” antigen, “tumour” antigen, transplanted tissue, “neo”
antigen, allergen
B cell antigens must be on pathogen surface
T cell antigens don’t need to be on pathogen surface
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Antigens inside pathogen are processed by antigen
presenting cells and loaded onto an antigen-presenting
Clonal Selection Hypothesis
oEach immune (receptor bearing lymphocyte) cell has a distinct, unique
receptor within it
oOnce the immune cell encounters its antigen, it is activated and begins
dividing into identical daughter cells which should destroy the pathogen
oAb molecules are secreted
Key Features of ad Adaptive Response
o(from the clonal selection hypothesis)
1) Each mature lymphocyte has many copies of a single antigen receptor
of unique specificity
2) Binding b/w antigen receptors on a lymphocyte and complementary
antigen is required or lymphocyte activation
3) The activated lymphocyte gives rise to daughters that have the same
antigen specificity as the parental cell
4) A lymphocyte whose antigen receptor specificity allows binding to
‘self ’ antigen is eliminated from the pool of mature, functional
oSpecificity of individual lymphocytes is a consequence of antigen receptor
structure and expression
oBCR and TCR proteins can distinguish b/w highly similar structures
oEach lymphocyte expresses thousands of antigen receptor proteins
oAll are encoded by the same gene (from same lymphocyte); identical
oClonal Selection Theory and Antigen Specificity of the Adaptive Response
oClonal Selection
Pool of lymphocyte clones of different specificity
Clone is selected due to activation via antigen
All responses from activation onwards are antigen specific,
so the response is antigen specific
oClonal Expansion
Clone is cloned (expansion)
Antigen specific daughter cells are the product of expanding clones
Plasma cells are fully differentiated cells that secrete the antibody
Called antigen-specific effector cells
oClonal selection of Antigen-specific T cells
oStart with T cell containing antigen specific TCR on its surface
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oForcing antigen  clonal selection and cell division + expansion
oDirect pathogen killing or help for B cells via proteins produced by T cells
oDiversification of B cells
oBone Marrow
Stem cell matures into pro-B cell
Pro-B cell expands (cloning)
Pre-B Lymphocyte
Expression of BCR on cell surface; important
developmental checkpoint
oBCR w/ high affinity for “self” = death
oBCR w/ low affinity for “self” = life
Mature B lymphocyte circulates in blood and lymphoid organs
seeking out its antigen
Plasma cell produces long term antibody secretion
Each B cell expresses a distinct BCR (10 billion unique B cells
in our blood)
oB cell Receptor
oContains two different genes;
V gene
C gene
oContains an antigen binding site, a heavy chain, and a light chain
The heavy chain is the constant site:
Dictates the class of antibody molecule
Only 5 classes in our bloodstream
oMembrane-bound Igs expressed as monomers
First isotype made during primary immune
Secreted form: pentamer
High valency
Best at binding antigens w/
repeating epitopes
Best at activating complement
Does not diffuse well
80% of serum Ig
cross placenta
modest complement activation
binding to Fc receptors for
opsonization of pathogens
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