The Immune System

Antigens

• Antigens are molecules (usually proteins or polysaccharides) found on the surface of cells.

• When a pathogen invades the body, the antigens on its surface are identified as foreign, which activates cells in the immune system.

• There are four main stages involved in the immune response:

Phagocytes Engulf Pathogens

A phagocyte is a type of cell that carries out phagocytosis. They are found in the blood and in the tissues and are the first cell to respond to a pathogen inside the body.

A phagocyte recognises the antigen on a pathogen.

The cytoplasm of the phagocyte moves around the pathogen, engulfing it.

This pathogen is now contained in a phagocytic vacuole in the cytoplasm of the phagocyte.

A lysosome fuses with the vacuole and it’s enzymes break down the pathogen.

The phagocyte then presents the pathogen’s antigens on its surface to activate other immune systems.

T-CellsPhagocytes activate T-

cells.

A T-cell is just a type of white blood

cell.

It recognizes foreign

antigens on the surface

of the phagocytes.

It has proteins on its surface which bind

to the antigen

carrying cells (pathogens).

This activates the

T-cell.

Different types of T-cell respond

in different ways:

Some release substances to activate B-cells (helper T-cells)

Some attach to antigens on the

pathogen and kill the cell.

(killer T-cells)

B-cells• B-cells are also a type of

white blood cell.

• They’re covered with antibodies (proteins that bind to an antigen to form an antibody-antigen complex)

• Each B-cell has a different shaped antibody on its membrane, which are all specific to a certain antigen.

When the antibody on the surface of the b-cell meets a complimentary antigen it binds to it

This, together with the substances T-cells release, activated the b-cells

The activated b-cell divides into plasma cells

Plasma CellsPlasma cells are identical

to B-cells.

They secrete loads of the

antibody specific to the

antigen.

Antibody functions include:

Coating the pathogen to

make it easier for the phagocyte to

engulf it

Coating the pathogen to

prevent it from entering host

cells

Binding to the antigen and neutralising

toxins produced by the antigen

1. Antibodies on the surface of b-cells. When these meet a complementary antigen, it will bind to it and be activated. When this happens, it is called an Antigen-antibody complex.

2. B-cells divide to form plasma cells (clones)

4 Binding to and neutralising toxins produced by the pathogen.

3. The plasma cells secrete loads of the antibody specific to the antigen.

The immune response is splitInto two…

Cellular: The T-cells and other immune system cells that

they interact with form the cellular

response

Humoral: B-cells and the production

of antibodies for the Humoral

response.

When an antigen enters the body for the first time it

activated the immune system, which is called the primary

response.

The primary response is slow because there aren’t many B-cells to produce the specific

antibodies to bind to the antigen.

Eventually the body will produce enough of the antibodies to

overcome the infection. Until then, the infected person will

suffer some of the symptoms of the disease.

After being exposed to an antigen, both T and B cells

produce memory cells, which trigger a much faster response if

that antigen enters the body again.

The person is now immune.

The Primary Response

The Secondary ResponseIf the same pathogen enters the

body again, the immune system will produce a quicker, stronger immune response- the secondary response.

Memory B-cells divide into plasma cells that produce the right antibody

to the antigen.

Memory T-cells divide into the correct type of T cells to kill the cell

carrying the antigen.

The secondary response often gets rid of the pathogen before you begin

to show any symptoms.

Exam QuestionsDescribe the function of antibodies. (3)

Antibodies coat pathogens, making it easier for phagocytes to engulf them. (1)And preventing them from entering host cells (2)They also bind to toxins to neutralise them (3)

Describe and explain how a secondary immune response differs to a primary immune response. (6)

A secondary immune response is faster (1)And produces a quicker, stronger response (2)Than the primary response. This is because memory cells are produced during the primary response (3)Which remember the foreign antigen (4)During the second infection, B memory cells can quickly divide to form plasma cells, which secrete the correct antibody to the antigen (5)Memory T cells quickly divide into the right type of t cells to kill the cell carrying the antigen (6)