topic 6.3- defence against infectious diseases

Question 1

describe the body’s primary defence mechanism

Answer 1

the skin and mucous membranes form a primary defence mechanism against pathogens that cause infectious disease

Question 2

skin

Answer 2

• outermost layer is tough and provides a physical barrier against the entry of pathogens and physical/chemical damage
• sebaceous glands are associated with hair follicles and secrete sebum, which maintains skin moisture and slightly lowering skin pH, which inhibits the growth of bacteria and fungi

Question 3

mucous membranes

Answer 3

thinner and softer type of skin found in airways and reproductive organs

• the mucus secreted is a sticky solution of glycoproteins and traps pathogens, which are either swallowed or expelled, acting as a physical barrier
• also has antiseptic properties due to the presence of the anti-bacterial enzyme lysozyme

Question 4

how are cuts in the skin sealed?

Answer 4

1. platelets aggregate at the site forming a temporary plug
2. they release clotting factors that trigger off the cascade of reactions involved in the clotting process
3. this cascade results in the production of thrombin, an enzyme, which converts the soluble protein fibrinogen into the insoluble fibrin
4. the resulting clot is initially a gel but if exposed to the air dries to form a hard scab

Question 5

Use of phagocytes in defence

Answer 5

• ingestion of pathogens by phagocytic white blood cells gives non-specific immunity to diseases
• they engulf of pathogens by endocytosis and digest them with lysosomes (enzymes)

Question 6

Use of antibody production in defence

Answer 6

Production of antibodies by lymphocytes in response to particular pathogens gives specific immunity

Question 7

define an antigen

Answer 7

any chemical that stimulates an immune response

Question 8

define a specific immune response

Answer 8

• the production of one type of antibodies specific to a particular pathogen’s antigens by lymphocytes

Question 9

describe the steps of antibody production

Answer 9

1. antigens on the pathogen stimulate cell division of the small group of lymphocytes that produce the appropriate antibody
2. plasma cells (large clones of lymphocytes) are produced within a few days and secrete large quantities of the antibody

Question 10

describe the role of antibodies

Answer 10

antibodies are large proteins that have two functional regions: a hyper variable region that binds to a specific antigen and another that helps the body fight the pathogen by

• making it more recognisable to phagocytes
• preventing viruses from docking to and entering host cells

Question 11

some of the lymphocytes produced during an infection are not active plasma cells but instead become

Answer 11

memory cells

Question 12

immunity to a disease involves

Answer 12

either having antibodies against the pathogens or memory cells that allow rapid production of the antibody

Question 13

describe the function of antibiotics

Answer 13

block processes that occur in prokaryotic cells but not eukaryotic cells. For example, bacterial DNA replication, transcription, translation, ribosome function and cell wall formation.

Question 14

what is the issue with widespread antibiotic use?

Answer 14

some strains of bacteria have evolved with genes which confer resistance to antibiotics and some strains have multiple resistance

Question 15

why can viral diseases not be treated using antibiotics?

Answer 15

Being non-living, they rely on the host cell’s enzymes for ATP synthesis and other metabolic pathways. These processes cannot be targeted by drugs as the host cell would also be damaged.

Question 16

medical name for a blood clot

Answer 16

thrombus

Question 17

coronary thrombosis is

Answer 17

the formation of blood clots in the coronary arteries - this can be a fatal condition.

Question 18

give 4 things that increase the risk of coronary thrombosis

Answer 18

coronary occlusion, damage to the capillary epithelium, hardening of the arteries, rupture of an atheroma

Question 19

Effects of HIV on the immune system and modes of transmission

Answer 19

Production of antibodies by the immune system is a complex process and includes different types of lymphocyte, including helper T-cells. The human immunodeficiency virus (HIV) invades and destroys helper T-cells. The consequence is a progressive loss of the capacity to produce antibodies.

In the early stages of infection, the immune system makes antibodies against HIV. If these can be detected in a person’s body, they are said to be HIV-positive.

HIV is a retrovirus that has genes made of RNA and uses reverse transcriptase to make DNA copies of its genes once it has entered a host cell. The rate at which helper T-cells are destroyed by HIV varies considerably and can be slowed down by using anti-retroviral drugs. In most HIV-positive patients antibody production eventually becomes so ineffective that a group of opportunistic infections strike, which would be easily fought off by a healthy immune system.

A collection of several diseases or conditions existing together is called a syndrome. When the syndrome of conditions due to HIV is present, the person is said to have acquired immune deficiency syndrome (AIDS).

AIDS spreads by HIV infection. The virus only survives outside the body for a short time and infection normally only occurs if there is blood to blood contact between infected and uninfected people. There are various ways in which this can occur:
- sexual intercourse, during which abrasions to the mucous membranes of the penis and vagina can cause minor bleeding
- transfusion of infected blood, or blood products such as Factor VIII
- sharing of hypodermic needles by intravenous drug users.

Question 20

Florey and Chain experiments

Answer 20

Florey and Chain tested penicillin on infected mice
Eight mice were injected with hemolytic streptococci and four of these mice were subsequently injected with doses of penicillin
The untreated mice died of bacterial infection while those treated with penicillin all survived – demonstrating its antibiotic potential