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The human immune system
A simplified account of how your
immune system works
The Humoral Response - Activation Phase
antigen
The immune response
starts with an antigen
approaching a cell of
the immune system,
here a macrophage.
macrophage
Antigen engulfed in vesicle
The macrophage
engulfs the antigen
by phagocytosis.
Lysosome
containing
enzymes
The vesicle containing an
antigen fuses with a
lysosome. The enzymes in the
lysozome break down the
antigen into fragments. This is
antigen processing.
antigen processing
Inside the macrophage,
the processed antigens
combine with special
enzymes and
proteins combining
class II MHC proteins.
These proteins can
move to the cell surface
membrane.
Class II
MHC proteins
The antigen/MHC protein
Processed antigen/
MHC protein complex
complex is displayed on the
immune cell surface
membrane. The
macrophage is now known
as an antigen presenting
cell.
Antigen-presenting cell
The next stage involves a helper T cell
(also know as a T-helper cell) as well as
the macrophage
receptors which bind to specific
antigen/MHC protein complex
T helper cell (TH)
macrophage (antigen-presenting cell)
The receptors on the helper T cell enable
it to bind to the specific antigen-MHC
complex of the antigen presenting cell.
helper T cell
macrophage (antigen-presenting cell)
The binding of the helper T cell with the
antigen - MHC protein complex triggers the
macrophage to release proteins (cytokines)
that activate the helper T cell.
Cytokines from
macrophage
The activated helper T cell now releases
its own cytokines
Cytokines
from helper
T cell
The released cytokines stimulate the
helper T cell to reproduce and form a
clone of cells. Each new cell has the same
receptors as the original helper T cell, so
they are specific for the original antigen.
Clone of helper T cells
The Humoral Response - Effector Phase
Another phase of the immune
response begins with a B cell.
IgM receptor
The B cell has membrane
bound globular receptor
antigen
proteins (called IgM). Some of
these are specific for the same
antigen presented earlier by
the antigen presenting cell.
B cell
The B cell’s receptor
protein (an IgM) binds to
the antigen, and the cell
engulfs the antigen by
endocytosis.
IgM bound to
antigen engulfed
by cell
lysosome
The vesicle formed
inside the B cell fuses
with a lysosome. This
contains digestive
enzymes which break
down the antigen.
Fused vesicles containing
antigen and enzymes from
lysosome
B cell
Fragments of the
digested antigen
remain after
processing within the
vesicle.
Processed antigen
Class II
MHC
protein
The processed antigen
is attached to Class II
MHC receptors within
the B cell, and is
transported to the
membrane.
Antigen/MHC protein complex
The MHC proteins form
a complex with the
antigen which is
displayed on the surface
of the B cell. It has
become another type of
antigen presenting cell
B cell (antigen presenting cell)
A helper T cell from the clone of cells
produced earlier specifically recognises
the antigen presented by the B cell.
Helper T cell clone
Antigen-presenting B cell
The helper T cell cell binds to the antigen/MHC
protein complex displayed by the B cell.
This triggers the release of cytokines from the T cell.
Once the cytokines are released the helper T cell no
longer binds to the B cell.
helper T cell binds to
antigen complex on B cell
Cytokines released by
helper T cell
The cytokines released by the helper T
cell stimulate the B cell to divide and form
a clone of identical cells
B cell clones
The B cells continue to
divide and form two
groups of clones. Some
are long – lived MEMORY
cells. Most are antibodysecreting PLASMA cells.
Plasma cells have
extensive endoplasmic
reticulum and many
ribosomes.
Memory
cell
Plasma cells
Plasma cells are essentially antibody
factories. They produce and secrete
antibodies identical to those of the surface
receptors of the original parent B cell
antibodies
Like the IgM surface receptors on the parent B cell,
the antibodies can bind to and inactivate the
antigens, forming an antibody-antigen complex.
This complex makes it easier for other white blood
cells to engulf the antigen (phagocytosis).
Antibody-antigen complex