2.4. Cell recognition and the immune system

Question 1

name 3 physical barriers to infection

Answer 1

• skin
• stomach acid
• gut and skin flora (competes with pathogens for food and space)

Question 2

definition of an antigen

Answer 2

a protein that is recognised as foreign by the body’s immune system

Question 3

what are 3 examples of non -specific responses to infection

Answer 3

• inflammation (histamines released by damaged white blood cells causing vasodilation)
• lysozyme action (enzymes found in the secretion of tears)
• interferon ( prevents protein synthesis in viruses)
• phagocytosis

Question 4

name the steps involved in phagocytosis

Answer 4

1. the phagocyte is attracted to the pathogen and binds to it.
2. the phagocyte has several receptors on its cell-surface membrane that attach to chemicals on the surface of the pathogen
3. the pathogen is engulfed and wrapped in a vesicle, forming a phagosome
4. Lysosomes within the phagocyte fuse with the phagosome. The hydrolytic enzymes stored inside, break down the pathogen.
5. the phagocyte then presents the pathogen’s antigens on its plasma membrane, to activate other immune cells= antigen presenting cell

Question 5

what is a specific immune response?

Answer 5

is antigen specific and produces responses specific to one type of pathogen only. This type of immune response relies on lymphocytes produced in the bone marrow

Question 6

what is the difference between B cells and T cells?

Answer 6

B cells mature in the bone marrow and are involved in the humoral response.
* T cells move from the bone marrow to the thymus gland where they mature, and are
involved in cell mediated response.

Question 7

what are memory cells?

Answer 7

Memory cells are cells which replicate themselves when exposed to an invading pathogen and remain in the lymph nodes for decades searching for the same antigen. This results in
a much faster immune response should the individual be infected by the same pathogen
again.

Question 8

what are B effector cells/ plasma cells?
what are T helper cells? what are T killer cells?

Answer 8

B effector or plasma cells are antibody producing cells.
* T helper cells stimulate B cells and T killer cells to divide.
* T killer cells destroy pathogen infected cells.

Question 9

when does the cell-mediated response take place?

Answer 9

the destruction of cells that have been infected or have become abnormal

Question 10

describe the response of T lymphocytes to a foreign antigen in the cell-mediated response?

Answer 10

1. Once a phagocyte has become an antigen-presenting cell it binds to and activates a helper T cell -> the T cell releases chemical signals and undergoes rapid mitosis
2. Rapid mitosis results in the production of identical (clonal) cells -> most are activated by binding to antigen presenting cells and some will develop into memory cells
3. Cytotoxic T cells bind to infected cells and release a protein called PERFORIN -> creates pores in the plasma membrane to increase permeablity of water = resulting in osmotic lysis of the infected cells

Question 11

what can the chemical signals released can activate?

Answer 11

• MORE phagocytosis
• the humoral immune response
• cytotoxic (Th) cells

Question 12

when does the humoral response occur?

Answer 12

the destruction of pathogens or toxins which have invaded the extracellular fluid

Question 13

describe the process of the humoral response?

Answer 13

1. B-cells in the extracellular liquid take up, process and express the antigens of a pathogen (antigen presenting cell )
   - antigens may be taken directly from the pathogen from another antigen-presenting cell or fluid
2. Antigen presenting B-cell binds to an activated helper T cell= causing the B-cell to undergo rapid mitosis
   = more identical cells produced
   = most develop into plasma cells, some will be memory
   – Plasma cells produce and secrete monoclonal antibodies specific to the pathogen’s antigen
3. Antibodies bind to the antigens, causing AGGLUTINATION (clumping) of the pathogen so they are more easily destroyed by phagocytosis

Question 14

definition of an antibody

Answer 14

proteins with binding sites that are specific to a particular antigen -> produced from monoclonal antibodies

Question 15

describe the structure of an antibody

Answer 15

These are made of four polypeptides chains (2 heavy and 2 light) forming a Y shaped structure. These are complementary to only a single antigen.
They work by forming an antigen-antibody complex which serve as markers for phagocytes to destroy attached cells. Due
to antibodies having two binding sites they can also clump cells together making them easier for phagocytes to find. This process is called agglutination.

Question 16

how are monoclonal antibodies used in:
- targeted medication
- medical diagnosis

Answer 16

Targeted medication- (e.g. anti cancer drugs) have particular antigens that are targeted by antibodies to measure levels of that antigen in the body

Medical diagnosis- (e.g. pregnancy tests) Monoclonal antibodies in home pregnancy kits are specific to the hormone human chorionic gonadotrophin.

Question 17

describe the role of memory cells in the primary immune response

Answer 17

1. the primary response is slow because there arent many B-cells to make the antibody needed to bind to it
2. the infected person will show symptoms but eventually the body will be able to produce enough of the right antibody to overcome this infection.
3. Both T- cells and B-cells produce MEMORY cells and remain in the body for a long time. Memory T-cells remember the specific ANTIGEN and memory B-cells record the SPECIFIC ANTIBODIES needed to bind to the antigen.
4. the person is now immune and can quickly respond to the second infection

Question 18

describe the role of plasma cells in the secondary immune response

Answer 18

1. the secondary response is quicker and stronger when the same pathogen re-enters the body
2. CLONAL SELECTION happens faster
   - Memory B-cells are activated and divided into plasma cells that produce the right antibody.
   - Memory T-cells are activated and divide into the correct T-cell to kill the cell carrying the antigen
3. person has zero symptoms as they are immune

Question 19

what are the differences between active and passive immunity?

Answer 19

ACTIVE- your immune system makes its OWN antibodies after being stimulated by an antigen
1) NATURAL- become immune after catching the disease
2) ARTIFICIAL- immune after being given a vaccination with a harmless dose of the antigen

PASSIVE- given antibodies made by a different organism and your immune system DOES NOT produce its own antibodies
1) NATURAL- a baby becomes immune due to its antibodies received from its mother, through placenta and breast milk
2) PASSIVE- after being injected with antibodies from someone else

Question 20

describe the structure of HIV

Answer 20

1) a core containing genetic material and the enzyme reverse transcriptase
2) a capsid
3) A viral/ lipid envelope (made of membrane)
4) Copies of attachment proteins that help HIV attach to the host helper T-cell

Question 21

in what cell does HIV replicate in? and how does HIV replicate itself?

Answer 21

T-HELPER CELLS
1) attachment proteins ATTACH to the RECEPTOR MOLECULE on the cell membrane of the host T-cell

2) the capsid released into the cell, where it UNCOATS and releases GENETIC MATERIAL (RNA) into the cell’s cytoplasm

3) inside the cell, reverse transcriptase is used to make a COMPLEMENTARY strand of DNA from the VIRAL RNA TEMPLATE

4) double stranded DNA is made and inserted into the human DNA

5) host cell enzymes make viral proteins from the viral DNA found within human DNA

6) viral proteins are ASSEMBLED into new viruses which BUD from the cell and infect other cells

Question 22

how does HIV cause the symptoms of AIDS?

Answer 22

AIDS occur when one has a failing immune system and their helper T-cell count drops
1) Initial symptoms include minor infections (e.g. in the nose, ears, genitals and respiratory)
2) As AIDS progresses the no of immune system cells decrease further and patients are susceptible to more serious infections
3) During the late stages, patients have a low number of immune system cells and can develop a range of serious infections
- Without treatment, AIDS can last around 10 years

Question 23

Why are antibiotics ineffective against viruses?

Answer 23

• antibiotics kill bacteria by interfering with metabolic reactions .They target bacterial enzymes and ribosomes -> different from human enzymes and ribosomes as antibiotics ONLY target bacterial cells not human ones
• Viruses don’t have their own enzymes or ribosomes- they use the ones in host cells so antibiotics CANNOT INHIBIT because they don’t target human processes
• Most anti-viral drugs are designed to target the few VIRAL-SPECIFIC ENZYMES
• Reverse-transcriptase inhibitors- drugs that can be designed to inhibit it WITHOUT AFFECTING the host cell

Question 24

what are the ethical issues surrounding vaccines?

Answer 24

1- ANIMAL TESTING- usually tested on animals before humans, some vaccines use animal based substances
2- TESTING VACCINES ON HUMANS-volunteers may be put at an unnecessary risk of contracting the disease, infected people have a greater gain than loss
3- SIDE EFFECTS- they are still protected by HERD IMMUNITY, some think its unfair
4- EPIDEMIC OF A NEW DISEASE- rush to receive it and a difficult decision as to who will receive it first

Question 25

what are the ethical issues surrounding monoclonal antibodies?

Answer 25

1- ANIMAL RIGHT ISSUES- animals are used to produce the cells from which monoclonal antibodies are produced

Question 26

what is an ELISA test used for?

Answer 26

Enzyme Linked Immunosorbent Assay ( ELISA)
- to see if a patient has any antibodies to a certain antigen to a certain antibody

Question 27

what is the main difference between a DIRECT ELISA and an INDIRECT ELISA?

Answer 27

DIRECT- uses a SINGLE antibody
- detects antigens

INDIRECT- uses TWO different antibodies
- detects antibodies

Question 28

describe the INDIRECT ELISA test for HIV

Answer 28

1- Sample antigen is added to a well
2- Patients antibodies are added to the well. Any HIV specific antibodies will bind to the HIV antigen stuck to the bottom of the well
3- Well is washed to remove any unbound antibodies
4- Secondary antibodies COMPLEMENTARY to the primary antibody is added to the well (enzyme attached) -> the secondary antibody can bind to the HIV-specific antibody (primary)
5- Well is washed again to remove unbound secondary antibodies
6- A solution is added to the well (containing a substrate) that can cause a reaction with the enzyme
7- If a coloured product is produced or changes colour, an antigen is present.