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Summary Cell recognition and immunity

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  • Unit 2 - Cells
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  • AQA

Summary notes for AQA A-level Biology Cell recognition and immunity. Includes clear information on the immune system, phagocytosis, lymphocytes, antibodies, vaccines, HIV and monoclonal antibodies. Summarised from class notes and the official course textbook. From an A* student.

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Document information

  • April 16, 2022
  • 8
  • 2021/2022
  • Summary

Subjects

  • immunity
  • immune system
  • pathogens
  • antigens
  • phagocytosis
  • lymphocytes
  • antibodies
  • memory cells
  • vaccines
  • hiv
  • monoclonal antibodies

Written for

  • A/AS Level
  • AQA
  • A Level Biology
  • Unit 2 - Cells
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Cell recognition and immunity



Defence mechanisms
• A pathogen is a micro-organism that is a parasite of an animal/plant that causes disease. These
include viruses, bacteria, fungi
• Pathogens cause disease by: infecting and killing body cells, producing toxins
• Immunity is the ability of an organism to resist infection by protecting cells against a specific
pathogen
• Antigens are proteins or glycoproteins present on the surface of cells and induce an immune
response

Self and non self
• Lymphocytes can distinguish substances from your body (self) and from outside (non-self)
• The antigens on the surface of cells are proteins with a specific tertiary structure which means
the immune system can identify different substances

How they recognise self antigens:
• In a foetus the lymphocytes collide with other cells
• Infection in the foetus is rare (mother and placenta) so the lymphocytes collide with self antigens
• Some will have receptors that are complementary to the self antigens, these ones die
• The remaining lymphocytes only respond to foreign material
• In adults the lymphocytes in the bone marrow only encounter self antigens and any ones that
show an immune response to these die before they mature

The body’s immune system can recognise the following things and then it produces an immune
response:

pathogens, toxins, abnormal body cells, cells from other organisms of the same species

Immune response
1. Physical or chemical barrier to entry
2. Non specific response: fever, inflammation, phagocytosis
3. Specific responses: provide long term immunity and a fast, large response.

Physical or chemical barriers
The body has a number of barriers to prevent entry of pathogens…

skin, hair, stomach acid, mucus, urine, tears

If these barriers fail, then there is a non specific response and/or a specific response


Phagocytosis
Involves the ingestion and digestion of pathogens by white blood cells called phagocytes.

1. Chemicals produced by the pathogen are
detected by phagocytes.
2. The phagocytes move to the chemicals by
chemotaxis
3. Phagocytes have receptors that recognise
and attach to the pathogen
4. The pathogen is ingested by endocytosis
5. Pathogen is surrounded by a phospholipid
bilayer and this is called a phagosome
6. Lysosome fuses with the phagosome and

, the enzymes (lysozymes) hydrolyse the pathogen’s cell wall and destroy it
7. Any useful substances are re-used by the cell - soluble substances absorbed by cytoplasm
8. Any indigestible material is removed by exocytosis

The phagocyte also presents a part of the pathogen on its cell surface and it becomes an antigen
presenting cell. This activates a specific response from the lymphocytes.


Lymphocytes
• Non self antigens are specific for the pathogen/chemical and produce a specific immune
response from lymphocytes
• There are two main types of lymphocyte:

B lymphocytes T lymphocytes
• Respond to pathogens or non self antigens • Respond to pathogens or non self antigens in
present in body fluids body cells (antigen presenting cells).
• Humeral immunity • Cell mediated immunity
• Produced in bone marrow (by stem • Produced in bone marrow (by stem cells) and
cells) and mature in bone barrow mature in thymus gland

3 types: 3 types:
• Plasma B cells • T helper cells
• Memory B cells • Cytotoxic T cells
• Dividing B cells • Memory T cells

B lymphocytes
• Humeral immunity
• Have specific protein receptors on their cell surface
• These receptors are a complementary shape (tertiary structure) to the non-self antigens on the
pathogen they are responding to
• Plasma B-cells produce specific antibodies that also have specifically
shaped binding sites that are complementary to the non-self antigen


Process…
1. B cell binds to the specific non-self antigen and pathogen is engulfed
by endocytosis
2. B cell becomes an antigen presenting cell and so a T cell binds
3. The B cell is activated to divide by mitosis to give a clone of the
plasma B cells (clonal selection)
4. Cloned B cells produce the specific monoclonal antibodies that are complementary to the
antigen on the pathogen and they destroy the pathogen
5. Some B cells develop into memory cells

Clonal selection = the process where the specific lymphocyte with the receptors that are
complementary to the non self antigens is chosen and is ‘cloned’


Antibodies
Antibodies are are globular proteins secreted by plasma B lymphocytes in response to the
presence of specific non-self antigens

• They are proteins so have amino acid monomers
• Produced by plasma B-cells - the antibody is specific related to the
receptors on their surface
• Composed of 4 polypeptide chains, 2 heavy and 2 light
• They have 2 identical specifically shaped antigen binding sites determined by

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