Unit 3 - Organisms exchange substances with their environment
Summary
Summary Notes- A Level Biology AQA 3.2.4 Cell recognition and the immune system
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Unit 3 - Organisms exchange substances with their environment
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AQA
Detailed notes on the specification point 3.2.4 which is 'Cell recognition and the immune system'. Notes are detailed but summarised so perfect for revision.
Unit 3 - Organisms exchange substances with their environment
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3.2.4 CELL RECOGNITION AND THE IMMUNE SYSTEM
ANTIGENS
An antigen is a speci c protein found on the outside of cells, that triggers an immune response.
Each antigen has its own unique shape. The more closely related 2 individuals are, the more antigens they have
in common.
Your body recognises the antigens on your cells as your own (self); anything with di erent antigens to you (non-
self) stimulates an immune response.
Antigens enable the immune system to identify:
pathogens (micro-organisms that cause disease)
cells from other organisms of the same species (e.g in transplanted organs)
abnormal body cells (e.g cancerous cells)
toxins (poisonous molecules often released by bacteria)
TYPES OF WHITE BLOOD CELLS:
PHAGOCYTES
- Made in bone marrow and is found in blood and in tissues
- Types;
• Neutrophils: engulf and digest pathogens/APCs
• Macrophages: Can punch holes in bacteria or stick proteins to the outside of the bacteria to make them more
appealing for the neutrophilic to destroy.
- Carries out phagocytosis= a non-speci c response
that destroys any non-self cell it detects
Stages of phagocytosis:
1) Any chemicals or debris released by pathogens or
abnormal cells attract the phagocytes and will
move towards these cells
2) The receptor binding site on the surface of the
phagocyte will attach to the ‘non-self’ antigens on
the pathogen
3) The phagocyte changes shape and engulfs the
pathogen and encloses it as a phagosome
4) The phagosome will fuse with a lysosome in the
phagosome and release H2O2, HCl and digestive
enzymes which hydrolyses the pathogen.
5) Harmless products removed (egested / excreted) by
exocytosis or used by phagocyte
6) Phagocytes which engulf the virus present some of the pathogens antigens on their own cell membrane-
becoming antigen presenting cells (APCs)- which initiates an immune response (T- cells)
LYMPHOCYTES
- Made in the bone marrow and B-cells also mature there but T-cells mature in the thymus
- They’re smaller than phagocytes but play an important role in the speci c immune response
- There are two types: T-Lymphocytes and B-Lymphocytes which are used in di erent immune responses;
1. Cell-mediated response - Involves specialised white blood cells called T-lymphocytes which target
pathogens inside and outside cells.
2. Humoral response - Involves specialised white blood cells called B-lymphocytes which target pathogens
by producing antibodies
➡ T-Lymphocytes
T-cell responses are described as a ‘cell mediated response’ because T-cells only respond to antigens which are
present on cells (APC), and not antigens detached from cells and within bodily uid (eg.blood)
There are three types of T-lymphocyte:
• Cytotoxic cells (TC cells) – destroy abnormal/infected cells, they release a protein (perforin) which embeds
in the cell surface membrane and makes a pore so any substances can enter/leave the cell, causing cell
death
• Helper cells (TH cells) – activate B-lymphocytes to produce antibodies.
• Suppressor cells – turn o immune response, e.g. turning o antibody production
Cell mediated response:
1. Once a pathogen has been engulfed and destroyed by a phagocyte (phagocytosis), the antigens are
positioned on the cell surface- becoming an APC
Ø fi ff fi ff fi fffl ff
, 2. Helper T-Cells have receptors on their surface which bind to the antigens
on APCs
3. Once attached, this activates the helper T-cells to divide by mitosis to
replicate and make a large number of clones
4. Cloned helper T cells di erentiate into di erent cells;
• Some remain helper T cells and activate B lymphocytes
• Some stimulate macrophages to perform more phagocytosis
• Some become memory cells for that speci c antigen
• Some become cytotoxic T cells (killer T cells)
➡ B-Lymphocytes
- Activated by Helper T cells to provide a humoral response
- The humeral response is the response involving B cells and
antibodies.
- ‘Humoral’= body uids- as antibodies are soluble and transport in it.
- Millions of types of B-lymphocyte cells are produced within us
because as they mature the genes coding for antibodies are
changed to code for di erent antibodies
Primary Immune Response;
1. When an antigen enters the body for the rst time, the small
numbers of B-lymphocytes with receptors complementary to that
antigen are stimulated to divide by mitosis.This is known as clonal
selection
2. Some of these B-lymphocytes become plasma cells that secrete lots
of antibody molecules (speci c to the antigen) into the blood, lymph
or linings of the lungs and the gut
3. The other B-lymphocytes become memory cells that remain
circulating in the blood for a long time
Secondary immune response
1. If the same antigen is found in the body a second time, the memory
cells recognise the antigen, divide very quickly and di erentiate into
plasma cells (to produce antibodies) and more memory cells
2. This response is very quick, meaning that the infection can be
destroyed and removed before the pathogen population increases
too much and symptoms of the disease develop
3. This response to a previously encountered pathogen is, relative to the
primary immune response, extremely fast
Antibodies::
- Glycoproteins, designed to recognise a speci c antigen (complimentary in shape)
- They work by forming an antigen-antibody complex which serve as markers for phagocytes to destroy attached
cells
-Due to antibodies having two binding sites they can also clump cells together for
phagocytes (agglutination)
Structure
•Y-shaped w/4 polypeptide chains- 2 long, heavy & 2 short, light chains.
•Have a constant region, the antibody uses this to bind to phagocytes.
•Have a variable region, speci c structure- antigen binding sites
•Have exible hinge regions, allow the branches of the Y to move away from each
other so can bind to multiple antigens
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