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IB Biology: Animal Physiology Summary
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Animal Physiology (Unit 11) Summary
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11.1 ANTIBODY PRODUCTIONAll nucleated cells of the body possess unique and distinctive surface molecules that
identify it as self. These self markers are called major histocompatibility complex
molecules (MHC class I) and function as identification tags. The immune system will
not normally react to cells bearing these genetically determined markers
(self-tolerance).
Any substance that is recognised as foreign and is capable of triggering an immune
response is called an antigen (non self). Antigens are recognised by lymphocytes
which bind to and detect the characteristic shape of an exposed portion (epitope).
Lymphocytes trigger antibody production (adaptive immunity) which specifically bind
to epitopes via complementary paratopes.
Antigenic determinants include:
★ Surface markers present on foreign bodies in the blood and tissue – including
bacterial, fungal, viral and parasitic markers
★ The self markers of cells from a different organism (this is why transplantation
often results in graft rejection)
★ Even proteins from food may be rejected unless they are first broken down
into component parts by the digestive system
A pathogen is an agent that causes disease – either a microorganism (bacteria,
protist, fungi or parasite), virus or prion. A disease is any condition that disturbs the
normal functioning of the body (i.e. the body can no longer maintain homeostasis).
,An illness is a deterioration in the normal state of health of an organism (a disease
may cause an illness).
Pathogens are generally species-specific in that their capacity to cause disease
(pathogenesis) is limited to a particular species. Polio, syphilis, measles and
gonorrhoea are examples of diseases caused by pathogens that specifically affect
human hosts.
Certain pathogens may cross the species barrier and be able to infect and cause
disease in a range of hosts. Diseases from animals that can be transmitted to
humans are called zoonotic diseases (or zoonoses).
Transmission of infectious diseases can occur via a number of distinct mechanisms:
★ Direct contact – the transfer of pathogens via physical association or the
exchange of body fluids
★ Contamination – ingestion of pathogens growing on, or in, edible food sources
★ Airborne – certain pathogens can be transferred in the air via coughing and
sneezing
★ Vectors – intermediary organisms that transfer pathogens without developing
disease symptoms themselves
When the body is challenged by a foreign pathogen it will respond with both a
non-specific and a specific immune reaction. Non-specific immune cells called
macrophages will engulf pathogens non-selectively and break them down internally.
A proportion of macrophages (dendritic cells) will present the antigenic fragments of
the pathogen to specific lymphocytes .
The body contains millions of different T lymphocytes and B lymphocytes that each
recognise a single, specific antigen. Antigenic fragments are presented to specific
helper T lymphocytes (TH cells) that, when activated, releases cytokines. The
cytokines stimulate a specific B cell that produces antibodies to the antigen to divide
and form clones (clonal selection). Most of the clones will develop into short-lived
plasma cells that produce large quantities of specific antibody. A small proportion of
clones will differentiate into long-lived memory cells that function to provide long-term
immunity.
Pathogens typically contain multiple distinct antigenic fragments on their surface and
hence a single pathogen is likely to stimulate several different T and B lymphocytes
to produce a variety of specific antibodies (polyclonal activation)
Antibodies aid in the destruction of pathogens by a number of different mechanisms:
★ Precipitation – Soluble pathogens become insoluble and precipitate
★ Agglutination – Cellular pathogens become clumped for easier removal
★ Neutralisation – Antibodies may occlude pathogenic regions (e.g. exotoxins)
, ★ Inflammation – Antibodies may trigger an inflammatory response within the
body
★ Complement activation – Complement proteins perforate membranes (cell
lysis)
Memory cells are produced to prevent this delay in subsequent exposures and
hence prevent disease symptoms developing. When a B lymphocyte is activated and
divides to form plasma cells, a small proportion will differentiate into memory cells.
Memory cells are long living and will survive in the body for many years, producing
low levels of circulating antibodies. If a second infection with the same pathogen
occurs, memory cells will react more vigorously to produce antibodies faster. As
antibodies are produced faster, the pathogen cannot reproduce in sufficient amounts
to cause disease symptoms. Hence, because pathogen exposure no longer causes
the disease to occur, the individual is said to be immune.
An allergen is an environmental substance that triggers an immune response despite
not being intrinsically harmful. This immune response tends to be localised to the
region of exposure (e.g. airways and throat) as an allergic reaction. A severe
systemic allergic reaction is called anaphylaxis and can be fatal if left untreated.
An allergic reaction requires a pre-sensitised immune state (i.e. prior exposure to the
allergen). When a specific B cell first encounters the allergen, it differentiates into
plasma cells and makes large quantities of antibody (IgE). The IgE antibodies attach
to mast cells, effectively ‘priming’ them towards the allergen. Upon re-exposure to
the allergen, the IgE-primed mast cells release large amounts of histamine which
causes inflammation.
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