Transcript Specific Resistance = Immunity

Humoral Immunity: B Cells
Each B cell produces a
different IgD&M on its
surface.
Some antigens require
involvement of helper T
cells for B cell activation.
Long-lived cells, stored in large numbers in
lymph nodes; host is now “primed” for any
second response by the same antigen.
(particularly when
stimulated by helper
T cell’s interleukin-2)
T-independent
versus
T-dependent
Antigens
• Antigens that are large
multivalent polymers that bind
multiple IgD&M on a B cell
surface will directly stimulating
humoral immunity independent of
any helper T cell involvement.
• Smaller protein antigens with
too few epitopes for multiple
IgD&M binding and direct
stimulation of B cells will require
involvement of helper T cells and
there interleukins; they are
dependent on T cells for
stimulating humoral immunity.
(see more below)
Antigens fall into one of two broad types!
(Protein; polysaccharide;
glycoprotein)
B cell
IgD & IgM
Immune
complex
forms:
ADCC:
1) Antibodies bind to
parasite surface.
2) Fc region
receptors on surface
of eosinophils,
macrophage and
natural killer cells
bind to antibody.
3) Release of
cytokinins, perforins
and other lytic
enzymes to damage
parasite tissues.
4) Liberation and /or
phagocytosis of
other parasite
antigens to further
activate immunity.
Cell-Mediated Immunity: T Cells
•
Helper T cells (TH) are needed to stimulate production of cytotoxic T cells
(TC) and humoral immunity (B cells) to T-dependent antigens.
•
Important to this system is the ability for lymphocytes to distinguish
normal host cells (“self”) from microbes or infected host cells. The major
histocompatibility complex (MHC) on the surface of all host cells
functions as a “self flag”. TH cell receptors (CD4+) bind to class II MHCs
and Tc cell receptors (CD8+) to class I MHCs.
•
Events in cell-mediated immunity:
1) Antigen presenting cells (APC; macrophage and dendrites)
phagocytize antigen carrying microbe, virus, or carrier molecule.
2) Antigen fragment with a unique epitope combines with the MHCs at
the APC surface for presentation to helper T cells. Simultaneously,
APC secrete IL-I.
3) A TH cell with a specific receptor for that epitope binds to the class II
MHC on the APC, which together with IL-1 results in proliferation of
that particular TH cell clonal population and secretion of other ILs.
Cell-Mediated Immunity: T Cells
4) The active clonal population of TH cells secretes IL-2 and other cytokinins.
5) These signals stimulate specific B cells to proliferate and mature to antibody
secreting plasma cells (see details below for T-dependent antigens).
6) These signals also stimulate specific Tc cells to proliferate and activate. The Tc
cells involved have their own receptors bound to class I - MHC.
The role of TH cells in
humoral immunity to
T-dependent antigens.
• TH cell receptors unique to the epitope
and MHC of concern may bind to B cells
with complementary surface features.
• These bound B cells are stimulated by
IL-2 to proliferate and mature to plasma
cells that secrete antibodies to the Tdependent antigen.
Cytotoxic T Cells:
1) They bind to infected target host cells (typically due to virus or
intracellular bacterium) expressing the antigen-MHC via Tc cell receptor.
They also bind to cells with non-self MHC (e.g. tranplants; tumor cells).
2) Binding stimulates secretion of extracellular enzymes (perforins) to
degrade the target cell plasma membrane.
3) Grazymes (proteolytic enzymes) are released by Tc cells and enter
infected cell through pores and induce apoptosis.
4) Rupturing of the membrane kills the target cell and releases Tc cell.
Immunity
Overview
Stages of Disease Development
Carrier?
Carrier?
Carrier?
Susceptible
to 2º infection
Carriers of Infectious Disease
Causative agent is Salmonella typhi