Adaptive Immunity: Specific Defenses of the Host
Innate immunity: Defenses against any pathogen
Adaptive immunity: Induced resistance to a specific pathogen
Historical Development
Pasteur observed immunity in chickens injected with weakened pathogens
Von Behring received the Nobel Prize for development of antitoxin
Ehrlich’s work led to the identification of antibodies in serum
Dual Nature of Adaptive Immunity
T and B cells develop from stem cells in red bone marrow
Humoral immunity
B cells mature in the bone marrow
Chickens: Bursa of Fabricius
Due to antibodies
Dual Nature of Adaptive Immunity
T and B cells develop from stem cells in red bone marrow
Cellular immunity
Due to T cells
T cells mature in the thymus
The Nature of Antigens
Antigen (Ag): A substance that causes the body to produce specific antibodies or
sensitized T cells
Antibodies (Ab) interact with epitopes or
antigenic determinants
Hapten: Antigen is combined with carrier
molecules
Antigens
Haptens
The Nature of Antibodies
Globular proteins called immunoglobulins
The number of antigen-binding sites determines valence
Antibodies
IgG Antibodies
Monomer
80% of serum Abs
Fix complement
In blood, lymph, and intestine
Cross placenta
Enhance phagocytosis; neutralize toxins and viruses; protects fetus and newborn
Half-life = 23 days
IgM Antibodies
Pentamer
5–10% of serum Abs
Fix complement
In blood, in lymph, and on B cells
Agglutinates microbes; first Ab produced in response to infection
Half-life = 5 days
IgA Antibodies
Dimer
10–15% of serum Abs
In secretions
Mucosal protection
Half-life = 6 days
IgD Antibodies
Monomer
0.2% of serum Abs
In blood, in lymph, and on B cells
On B cells, initiate immune response
Half-life = 3 days
IgE Antibodies
Monomer
0.002% of serum Abs
On mast cells, on basophils, and in blood
Allergic reactions; lysis of parasitic worms
Half-life = 2 days
Activation of B Cells
Major histocompatibility complex (MHC) expressed on mammalian cells
T-dependent antigens
Ag presented with (self) MHC to TH cell
TH cell produces cytokines that activate the B cell
T-independent antigens
Stimulate the B cell to make Abs
Activation of B Cells
Clonal Selection
Activation of B Cells
B cells differentiate into
Antibody-producing plasma cells
Memory cells
Clonal deletion eliminates harmful B cells
The Results of Ag-Ab Binding
Agglutination
Opsonization
Complement Fixation
Antibody-Dependent Cell-Mediated Immunity
Neutralization
T Cells and Cellular Immunity
T cells mature in the thymus
Thymic selection eliminates many immature T cells
T cells respond to Ag by T-cell receptors (TCRs)
T cells require antigen-presenting cells (APCs)
Pathogens entering the gastrointestinal or respiratory tracts pass through
M (microfold) cells over
Peyer’s patches, which contain APCs
M Cells on Peyer’s Patch
M Cells
T Helper Cells
CD4+ or TH cells
TCRs recognize Ags and MHC II on APC
TLRs are a costimulatory signal on APC and TH
TH cells produce cytokines and differentiate into
TH1
TH2
Memory cells
T Helper Cells
TH1 produces IFN-which activates cells related to cell-mediated
immunity, macrophages, and
Abs
TH2 activate eosinophils and B cells to produce
IgE
Activation of CD4+ T Helper Cells
T Cytotoxic Cells
CD8+ or TC cells
Target cells are self carrying endogenous antigens
Activated into cytotoxic T lymphocytes (CTLs)
CTLs recognize Ag + MHC I
Induce apoptosis in target cell
CTL releases perforin and granzymes
T Cytotoxic Cells
Apoptosis
T Regulatory Cells
Treg cells
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CD4 and CD25 on surface
Suppress T cells against self
Antigen-Presenting Cells
Digest antigen
Ag fragments on APC surface with MHC
B cells
Dendritic cells
Activated macrophages
A Dendritic Cell
Activated Macrophages
Natural Killer (NK) Cells
Granular leukocytes destroy cells that don’t
express MHC I
Kill virus-infected and tumor cells
Attack parasites
ADCC
Antibody-dependent cell-mediated cytotoxicity
ADCC
Cytokines
Chemical messengers
Overproduction leads to cytokine storm
Cells Communicate via Cytokines
Cells Communicate via Cytokines
Immunological Memory
Antibody titer is the amount of Ab in serum
Primary response occurs after initial contact with Ag
Secondary (memory or anamnestic) response occurs after second exposure
Immune Responses to an Antigen
Types of Adaptive Immunity
Naturally acquired active immunity
Resulting from infection
Naturally acquired passive immunity
Transplacental or via colostrum
Artificially acquired active immunity
Injection of Ag (vaccination)
Artificially acquired passive immunity
Injection of Ab
Terminology of Adaptive Immunity
Serology: The study of reactions between antibodies and antigens
Antiserum: The generic term for serum because it contains Ab
Globulins: Serum proteins
Immunoglobulins: Antibodies
Gamma (g)
globulin: Serum fraction containing Ab