The human immune system

A simplified account of how your immune system works

The immune response

starts with an antigen

approaching a cell of

the immune system,

here a macrophage.

antigen

macrophage

The Humoral Response - Activation Phase

The macrophage

engulfs the antigen

by phagocytosis. Lysosome containing enzymes

Antigen engulfed in vesicle

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

class II MHC proteins.

These proteins can

move to the cell surface

membrane.

enzymes andproteins combining

Class IIMHC proteins

The antigen/MHC protein

complex is displayed on the

immune cell surface

membrane. The

macrophage is now known

as an antigen presenting

cell.

Processed antigen/MHC protein complex

Antigen-presenting cell

The next stage involves a helper T cell

(also know as a T-helper cell) as well as

the macrophage

T helper cell (TH)

macrophage (antigen-presenting cell)

receptors which bind to specificantigen/MHC protein complex

The receptors on the helper T cell enable

it to bind to the specific antigen-MHC

complex of the antigen presenting cell.

macrophage (antigen-presenting cell)

helper T 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

Another phase of the immune

response begins with a B cell.

The B cell has membrane

bound globular receptor

proteins (called IgM). Some of

these are specific for the same

antigen presented earlier by

the antigen presenting cell.

The Humoral Response - Effector Phase

antigen

B cell

IgM receptor

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 IIMHCprotein

The processed antigen

is attached to Class II

MHC receptors within

the B cell, and is

transported to the

membrane.

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

Antigen/MHC protein complex

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.

Antigen-presenting B cell

Helper T cell clone

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.

Cytokines released by helper T cell

helper T cell binds to antigen complex on B 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 antibody-

secreting PLASMA cells.

Plasma cells have

extensive endoplasmic

reticulum and many

ribosomes.

Memorycell

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