C3.2 Defence against disease Flashcards
C3.2.1—Pathogens as the cause of infectious diseases
Students should understand that a broad range of disease-causing organisms can infect humans. A
disease-causing organism is known as a pathogen, although typically the term is reserved for viruses,
bacteria, fungi and protists. Archaea are not known to cause any diseases in humans.
NOS: Students should be aware that careful observation can lead to important progress. For example,
careful observations during 19th-century epidemics of childbed fever (due to an infection after childbirth)
in Vienna and cholera in London led to breakthroughs in the control of infectious disease.
C3.2.2—Skin and mucous membranes as a primary defence
The skin acts as both a physical and chemical barrier to pathogens. Students are not required to draw or
label diagrams of skin.
C3.2.3—Sealing of cuts in skin by blood clotting
Include release of clotting factors from platelets and the subsequent cascade pathway that results in rapid
conversion of fibrinogen to fibrin by thrombin and trapping of erythrocytes to form a clot. No further
details are required.
C3.2.4—Differences between the innate immune system and the adaptive immune system
Include the idea that the innate system responds to broad categories of pathogen and does not change
during an organism’s life whereas the adaptive system responds in a specific way to particular pathogens
and builds up a memory of pathogens encountered, so the immune response becomes more effective.
Students are not required to know any components of the innate immune system other than phagocytes.
C3.2.5—Infection control by phagocytes
Include amoeboid movement from blood to sites of infection, where phagocytes recognize pathogens,
engulf them by endocytosis and digest them using enzymes from lysosomes.
C3.2.6—Lymphocytes as cells in the adaptive immune system that cooperate to produce antibodies
Students should understand that lymphocytes both circulate in the blood and are contained in lymph
nodes. They should appreciate that an individual has a very large number of B-lymphocytes that each
make a specific type of antibody.
C3.2.7—Antigens as recognition molecules that trigger antibody production
Students should appreciate that most antigens are glycoproteins or other proteins and that they are
usually located on the outer surfaces of pathogens. Antigens on the surface of erythrocytes may stimulate
antibody production if transfused into a person with a different blood group.
C3.2.8—Activation of B-lymphocytes by helper T-lymphocytes
Students should understand that there are antigen-specific B-cells and helper T-cells. B-cells produce
antibodies and become memory cells only when they have been activated. Activation requires both direct
interaction with the specific antigen and contact with a helper T-cell that has also become activated by the
same type of antigen.
C3.2.9—Multiplication of activated B-lymphocytes to form clones of antibody-secreting plasma cells
There are relatively small numbers of B-cells that respond to a specific antigen. To produce sufficient
quantities of antibody, activated B-cells first divide by mitosis to produce large numbers of plasma B-cells
that are capable of producing the same type of antibody
C3.2.10—Immunity as a consequence of retaining memory cells
Students should understand that immunity is the ability to eliminate an infectious disease from the body.
It is due to the long-term survival of lymphocytes that are capable of making the specific antibodies
needed to fight the infection. These are memory cells.
C3.2.11—Transmission of HIV in body fluids
Include examples of the mechanisms of HIV (human immunodeficiency virus) transmission.
C3.2.12—Infection of lymphocytes by HIV with AIDS as a consequence
Students should understand that only certain types of lymphocyte are infected and killed, but that a
reduction in these lymphocytes limits the ability to produce antibodies and fight opportunistic infections.
C3.2.13—Antibiotics as chemicals that block processes occurring in bacteria but not in eukaryotic cells
Include reasons that antibiotics fail to control infection with viruses.
C3.2.14—Evolution of resistance to several antibiotics in strains of pathogenic bacteria
Students should understand that careful use of antibiotics is necessary to slow the emergence of
multiresistant bacteria.
NOS: Students should recognize that the development of new techniques can lead to new avenues of
research; for example, the recent technique of searching chemical libraries is yielding new antibiotics.
C3.2.15—Zoonoses as infectious diseases that can transfer from other species to humans
Illustrate the prevalence of zoonoses as infectious diseases in humans and their varied modes of infection
with several examples including tuberculosis, rabies and Japanese encephalitis. Include COVID-19 as an
infectious disease that has recently transferred from another species, with profound consequences for
humans.
