Unit 12: Diseases and Infection Learning Aim D: Understand how the human body responds to diseases and infections. This assignment and unit was graded an overall DISTINCTION. It is an extremely detailed assignment that meets all criteria in the best of detail, everything has been analysed, explaine...
What is the immune system?
The immune system is a complex network of organs, cells, tissues and proteins that defends
the body against foreign substances such as viruses, bacteria, fungi, parasites, protozoa and
toxins, whilst protecting the body's own cells.
The immune system can be divided into 3 lines of defences in the human body:
1. First line of defence: these are the surface barriers that prevent pathogens from
entering the body, they include:
Physical barriers: Physical barriers help eliminate pathogens and prevent tissue and blood
infections. The skin is one of a physical barrier, it acts as a primary barrier which forms a
tough and impermeable barrier to pathogens. The mucosa that lines the digestive,
reproductive and respiratory tracts also acts as a barrier by trapping and blocking pathogens
by producing mucus. Hairs inside the nasal cavity as well as the earwax found in ears also
trap pathogens and pollutants caused by the environment.
Chemical barriers: chemical barriers consist of various substances and chemicals that help
protect the body against pathogens. Several enzymes in the body such as lysosomes which
are found in tears, sweat and saliva have antimicrobial properties which weaken, kill, limit
growth and damage DNA and proteins of pathogens. Some acidic fluids such as gastric acid,
urine, vaginal secretion have a low acidic pH which inhibits the growth and survival of
pathogens. Sebum is an oily substance that is secreted by glands in the skin which forms a
protective layer on the skin’s surface, helping it to maintain its strength and integrity as well
as preventing the growth of microorganisms.
2. Second line of defence: this is the innate form of non-specific defence that removes
pathogens that have entered the body after going through the first line of defence.
They include:
Antimicrobial proteins: the body creates a lot of antimicrobial proteins such as interferons
and complement proteins. Interferons are antiviral glycoproteins that are released by living
cells in response to viral attacks and activate a viral resistance state to closeby cells.
Complement proteins enhance phagocytosis, directly kill the pathogen creating pores on the
membrane and causing lysis of the pathogen and also triggers the inflammatory response
Fever: injured cells release chemicals that tell the brain to increase in core body
temperature above 38 degrees. This produces a fever that helps slow down or even kills
some pathogens. As the body recovers from the infection, the cells stop releasing the
signalling chemicals. It also speeds up processes that help the immune system deal with
threat
Inflammation: the body initiates inflammatory response if there is an infection or some
tissues have been damaged. This creates a lot of blood flow to the infection site which leads
, to painful redness and swelling around the site of infection. Inflammation helps to find where
the infection is, collect immune cells to help fight the infection and heal the infection.
Phagocytes: there are many phagocytes which all find and kill pathogens or substances.
Neutrophils specifically attack bacteria, they quickly get to the site of the bacteria and kill
them but they can also be killed in return. They only last a few days in the body before they
self-destruct but our bone marrow produces a lot more each day. T helper cells produce
signals to give instructions to cells. Each T helper cell specifically targets one type of
pathogen. Cytotoxic killer T cells create holes in the walls of pathogens so that their cell
content seeps out. Macrophages ingest or clean up dead cells. Dendritic cells observe if
there is an invader and present them to lymphocytes to initiate and regulate the adaptive
immune response. B cells produce antibodies which bind to the antigen of the invading
bacteria and prevent them from moving until the macrophages ingest them. B cells can
sometimes turn into memory cells which can help recognise that antigen again. Suppressor
T cells slow down the immune system to prevent damage to healthy cells
➔ Phagocytosis: phagocytosis is the process of ingesting bacteria or other materials by
phagocytes. The steps of phagocytosis include:
1. Chemotaxis: chemotaxis is the movement of an organism or cell in response to a
chemical stimulus. It involves chemical signals being released by immune or
damaged cells in order to attract phagocytes to the site. The phagocytes then identify
these chemical signals and travel towards the site. This process is called chemotaxis
2. Binding and recognition: the phagocytes recognise and bind to pathogens and
other substances as soon as it has reached the site. This binding and recognition
occurs through receptors and MHC molecules on the surface of the particle
3. Ingestion: after the phagocytes attach and bind to the pathogen, the phagocyte
stretches its cell membrane around the invader and forms a phagosome ensuring
that the particle is inside the cytoplasm of the phagocyte so it can’t escape
4. Formation of phagolysosome: the phagosome binds with lysosomes to for a
phagolysosome which contains the ingested particle and destructive enzymes
5. Destruction: the enzymes break down the ingested particle into smaller pieces, this
all happens in the phagolysosome. This process helps to neutralise the particle and
stop it from replication and surviving in the body
6. Elimination: once the particle has been neutralised, the phagolysosome forms a
residual body that contains the broken down products from the phagolysosome and
is eventually eliminated from the cell.
3. Third line of defence: these are specific lymphocytes that produce antibodies as
part of the adaptive immune response. They include:
Lymphocytes: lymphocytes are a type of white blood cell that help your body’s immune
system fight pathogens and cancer. There are two types of lymphocytes involved in the third
line of defence:
1. B lymphocytes: a B lymphocyte contains a B cell receptor, these are made up of
antibodies that identify and bind to specific antigens. They also contain
immunoglobulins which can move around in the bloodstream and bodily fluids so that
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