Comprehensive and in-depth notes on the biological aspects of this chapter, using specification points as headings to ensure that all required material is included- and no irrelevant content (like many of the textbooks). Created and used by an A-Level Biology student for the NEW SPECIFICATION from ...
Biology Spec Led Revision
Chapter 11 - Disease Prevention
Defences Against Infection in Animals
Vertebrate animals have a sophisticated set of defences against invasion by pathogens. There are
4 main ways in which animals defend themselves against infectious diseases:
1) Physical: Tissue proved barriers that pathogens cannot pass through unaided, i.e. skin
and mucous membranes that line the alimentary canal, the gas exchange system and the
urogenital tract.
2) Cellular: Cells alert the body to the presence of pathogens, produce substances that
provide protection and ingest and digest pathogens
3) Chemical: Substances secreted by the body to produce inhospitable environments for
pathogens, trap them, cause them to burst, stop them reproducing or growing, or stopping
them from entering cells.
4) Harmless Bacteria and Fungi: Commensal organisms that live on and inside of us,
which compete with pathogenic organisms to prevent infections.
There is 3 lines of defence, each consisting of a mixture of cellular and chemical defences
FIRST LINE OF DEFENCE - PHYSICAL BARRIERS
FEATURES OF THE SKIN:
• The skin and mucous membranes are very effective first line of defence
• The outer layer of skin consists of DEAD CELLS filled with KERATIN, which is a TOUGH
FIBROUS PROTEIN
• This layer of dead, hardened cells is relatively DRY, and skin secretions make the surface
ACIDIC
• When sweat evaporates, SALT is left behind on the skin
• Conditions of LOW MOISTURE, ACIDITY, and HIGH SALINITY prevent micro-organisms from
growing and multiplying on the skin
MUCOUS MEMBRANES:
• The gut, airways and reproductive system are lined with MUCOUS MEMBRANES that consist of
EPITHELIAL CELLS interspersed with MUCOSE SECRETING CELLS, i.e: Goblet Cells
• Mucous is a sticky, slimy substance which is full of GLYCOPROTEIN MOLECULES, each having
LONG HYDROCARBON chains which make it sticky
• Small particles in the air, such as bacteria, viruses, dust and pollen are trapped in the mucous
and then MOVED UP by the CILLIA in the trachea to the back of the throat
• Here it is either EXPELLED or enters the DIGESTIVE system by going into the stomach
EXPULSIVE REFLEXES:
• Sometimes pathogens IRRITATE the linings of the airways, promoting a sneeze or cough
• Sudden expulsions carry secretions from the respiratory tract including foreign particles that
have entered
• They are expulsive reflexes which eject pathogens from the upper airway, increasing the
chances of being transmitted
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