8 terms

6.3 Defence against infectious disease

6.3.1 Define pathogen.
A pathogen is an organism that can cause disease.
Pathogens include bacteria, viruses, protista, fungi and other parasitic multicellular organisms.
6.3.2 Explain why antibiotics are effective against bacteria but not against viruses.
Antibiotics block metabolic pathways in bacteria: i.e. inhibit cell wall formation;
protein synthesis.
Viruses use host cell metabolic pathways;
Viruses do not possess a cell wall and so
are not affected by antibiotics.
Antibiotics are not used to treat viral diseases because they are ineffective and may harm helpful bacteria.
6.3.3 Outline the role of skin and mucous membranes in defence against pathogens.
The skin and mucous membrane act as a physical barrier;
Skin has several layers of keratinized cells (dead cells);
The skin is dry discouraging the growth and reproduction of pathogens;
Skin and mucous membrane host natural flora and fauna which compete with pathogens;
The enzyme lysozyme is present on the skin's surface to break down pathogens;
The pH of skin and mucous is unfavourable to many pathogens;
Skin is a continuous layer;
Mucus traps pathogens and is sticky.
6.3.4 Outline how phagocytic leucocytes ingest pathogens in the blood and in body tissues.
The second line of defence is the non-specific immune system, a host of quick, non-specific methods of killing microbes that have passed the first line of defence and entered the body.
Phagocytes are large, irregularly-shaped leukocytes that destroy bacteria, viruses, and dust particles.
The phagocytes show amoeboid movement (foot like structure), constantly changing shape, as they engulf microbes.
The engulf vesicles join together to form phagosomes(phagocytic vesicle).
The phagosome then fuses with lysosomes which contain containing lysozymes.
These enzymes killing and digesting the microbes.
The process is called phagosytosis.
6.3.5 Distinguish between antigens and antibodies.
Antigen is a molecule that causes antibody formation;
Antibody is a (globular) protein or molecule that recognizes an antigen.
6.3.6 Explain antibody production.
(a) There are many different lymphocytes.
(b) The antigen infects and is presented to the lymphocytes
(c) The lymphocyte with a surface epitope complementary to the antigen is selected.
(d) The Lymphocyte clones to produce many plasma cells. This occurs in the lymph nodes.
(e) The clone of plasma cells
(f) The gene for the antibody is expressed and secreted into the plasma and tissue fluid.
(g) The antibody circulated in body fluids destroying the infectious antigen.
6.3.7 Outline the effects of HIV on the immune system.
HIV attacks T-cells which are part of the immune system that are important for the formation of B-Cells.
The virus enters the T-Cells and replicates there. As reproduction increases, the cell breaks up and the virus RNA is spread to other T-cells.
The virus keeps infecting and killing other T-cells, paralyzing the immune system.
This enables other organisms usually kept under control by the immune system to be able to affect the body.
6.3.8 Discuss the cause, transmission and social implications of AIDS.
human immunodeficiency virus(HIV);
Retrovirus from RNA to DNA;
Enters T-helper cells;
Immune system becomes weakened;
Greater chance for opportunistic infections;

sexually transmitted;
can be transmitted from human to human or mother to fetus;
From breast milk, saliva and other body fluids;
Use of dirty needles;
Blood transfusions;

social implications of AIDS:
Many orphaned children;
Social discrimination;
Problems obtaining employment, life insurance;
Impact and costs on health systems of treating people;
early death reduces number of adults, reduces workforce, reduces family income;
Drug treatment expensive;
Encourages use of condoms;