COURSE OF THE ADAPTIVE IMMUNE RESPONSE Flashcards
what are the phases of humoral immunity?
- antigen recognition
- activation of B lymphocytes
- clonal expansion and differentiation
- effector functions
explain antigen recognition.
Naïve B lymphocytes express two classes of membrane bound
antibodies IgM and IgD which function as the receptors for
antigens.
signalling requires cross-linking of at least two Ig receptors
Polysaccharides, lipids and other non protein antigens often contain
multiple identical epitopes in each molecule and are therefore able
to bind to numerous Ig receptors on a B cell at the same time.
Signals initiated by antigen receptor cross-linking are transduced by
Ig-associated proteins Igα and Igβ.
what is naive B lymphocyte?
a B lymphocyte that is not activated as it is not bind with an antigen yet.
when will receptors cross linking occurs?
Receptor cross-linking occurs when two or more antigen molecules
in an aggregate or repeating epitopes of one antigen molecules bind
to adjacent Ig molecules in the membrane of B cells
explain activation of B lymphocyte
There are two routes from B cell activation to production of
antibodies:
- activation by Thymus dependent antigens (T-dependent antigens)
- activation by Thymus independent antigens (T-independent
antigens).
what does it mean by thymus independent antigens?
it didnt require T helper cell
explain activation of B lymphocyte by Thymus dependent antigens (T-dependent antigens).
most antigens requires for T helper cells to induce antibody
production.
Binding of Ag to B-cell does not induce proliferation and differentiation to effector cells without additional interaction with T
helper cells and the presence of appropriate cytokines.
explain the activation of B lymphocyte’s pathway
The pathway:
- Antigen crosslinks two Ig receptors thus generate signal 1 - The B cell processed the antigen – the peptide are bound by MHC class II and presented on the membrane as peptide-MHC complex.
- TH cells recognize peptide-MHC class II on B-cell membrane – this activates TH cell.
- TH cell begins to express CD40 ligand (CD40L) – interaction of CD40 and CD40L generate signal 2.
- B cell begins to express receptors for various cytokines - cytokines produced by TH cells bind to cytokines receptor on B cell and stimulate more B cell proliferation and antibody production.
explain activation of B lymphocyte by Thymus independent antigens (T-independent antigens).
antigens can directly stimulate the B cells to produce antibody without the requirement for T cell help.
Example of antigens are lipopolysaccharides & antigens with highly repetitious molecules.
The humoral response to TI Ag is generally weak; no memory cells
formed.
explain clonal expansion and differentiation.
The transcription factors is activated – produce proteins that involved in B cell proliferation (clonal expansion) and differentiation – B cells differentiate into effector cells (plasma cells that secrete antibodies) and memory cells.
once activated, B cell may generate up to 4,000 antibody –secreting
cells which can produce up to 1012 antibody molecules per day.
The secreted antibodies have the same specificity as the naïve B cell
membrane receptors that recognized antigen to initiate the response.
Repeated exposure to a protein antigen results in the production of
antibodies with increasing affinity for the antigen- affinity maturation.
This leads to the production of antibodies with improved capacity to
bind and neutralize microbes and their toxins.
explain effector functions.
Effector functions of antibodies.
Antibodies do not kill or eliminate antigens.
Their roles are to bind to antigens and mediate effector functions.
The binding of antibody to antigen can result in several mechanism.
explain the mechanism of binding antibodies to antigens: agglutination
- Antibodies with multiple antigen
binding sites cause agglutination of
microbes thus reducing the chances of spread throughout the body. - Agglutination cause more effective
phagocytosis.
explain the mechanism of binding antibodies to antigens: opsonization and promotion of phagocytosis
Antibodies coat microbes and
promote their ingestion by
phagocytosis.
explain the mechanism of binding antibodies to antigens: complement activation
Formation of immune complex can
trigger complement cascade reaction
which can lead to formation of pores
and cell lysis.
explain the mechanism of binding antibodies to antigens: neutralization of microbes and microbial toxins
Antibody bind to and
block/neutralize the infectivity of
microbes and the interactions of
microbial toxins with host cells.
explain the mechanism of binding antibodies to antigens: antibody dependent cell-mediated cytotoxicity
- the linking of antibody bound with
target cells with the FC receptors can
induce/direct the cell cytotoxic activity of the effector cells against the target cells (lysis of the target cell). - A special type of ADCC mediated by
eosinophils plays a role in defense
against helminthic infections.
explain the mechanism of binding antibodies to antigens: immobilization of bacteria and protozoans
antibodies attach to cilia and flagella, thus the structure will be
less active – easier for phagocytes to engulf.
what is antibody activation of immune cells?
antibody response to the first exposure to an antigen is called
primary immune responses and that to the subsequent exposures
are called secondary immune responses.
what are the types of humoral responses
- primary immune response
- secondary immune response
explain primary immune response.
Immediately after the first exposure to an antigen there is a duration called the lag period during which naïve B cells undergo clonal selection and expansion and differentiation into plasma cells and memory cells.
The lag period is followed by a
logarithmic increase in serum antibody level which reaches a peak, plateaus for a variable time and the declines.
the duration of the lag time varies with the nature of the antigen.
The antibody isotype in primary immune response is mainly IgM.
Memory B cells formed during primary immune response stop dividing and enter the G0 phase of the cell cycle - these cells have variable life span.
explain secondary immune response.
The response results from activation
of memory cells.
They differ both qualitatively and
quantitatively.
The amount of antibody produced
after the first encounter with an
antigen are smaller than amounts of
antibody in secondary immune
response.
Secondary responses also showed
increase antibody affinity.
The lag period is shorter than in
primary immune response.
The antibody isotype in secondary
immune response is IgG and under
certain situations IgE and IgA.
