Topic 6.7 Response to infection

Question 1

What is an antigen?

Answer 1

Cell-surface molecule that can stimulate immune response.

Usually (glyco)protein, sometimes (glyco)lipid or polysaccharide.

Immune system recognises as “self” or “no-self” = enables identification of cells from other organsisms of same species, pathogens, toxins and abnormal body cells.

Question 2

Outline the process of inflammation

Answer 2

1. Damaged vessles release histamines, causing vasodialtation
2. Blood flow and permeability of blood vessels increase
3. White blood cells and plasma into the infected tissue

Question 3

Name the two types of white blood cell involved in phagocytosis

Answer 3

Neutrophils

Macrophages (can become antigen-presenting cells)

Question 4

How does phagocytosis destroy pathogens?

Answer 4

1. Phagocyte moves towards pathogen via chemotaxis.
2. Phagocyte engulfs pathogen via endocytosis to form a phagosome.
3. Phagosome fuses with lysosome (phagolysosome).
4. Lysozymes digest pathogen.
5. Phagocyte absorbs the products from pathogen hydrolysis.

Question 5

Explain the role of antigen-presenting cells (APCs)

Answer 5

Macropage displays antigen from pathogen on its surface (afterhydrolysis in phagocytosis).

Enhances recognition by T_H cells, which cannot directly interface with pathogens/ antigens in body fluid.

Question 6

Give two differences between specific and nonspecific immune responses

Answer 6

Nonspecific (inflamation, phagocytosis) = same for all pathogens.
Specific (B and T lymphocytes) = complementary pathogen.

Nonspecific = immediate
Specific = time lag

Question 7

Name the two types of specific immune response

Answer 7

• Cell-mediated
• Humoral

Question 8

Outline the process of the cell-mediated response

Answer 8

1. Complementary T_H lymphocytes bind to foreign antigen on APC
2. Stimulates:
   a. Clonal expansion of complementary T_H cells (rapid mitosis) become memory cells or trigger humoral response.
   b. Clonal expansion of cytotoxic T cells (T_C): secrete enzyme perforin to destroy infected cells.

Question 9

Outline the process of the cell-mediated response

Answer 9

1. Complementary T_H lymphocytes bind to foreign antigen on APC
2. Stimulates:
   a. Clonal expansion of complementary T_H cells (rapid mitosis) become memory cells or trigger humoral response.
   b. Clonal expansion of cytotoxic T cells (T_C): secrete enzyme perforin to destroy infected cells.

Question 9

Outline the process of the cell-mediated response

Answer 9

1. Complementary T_H lymphocytes bind to foreign antigen on APC
2. Stimulates:
   a. Clonal expansion of complementary T_H cells (rapid mitosis) become memory cells or trigger humoral response.
   b. Clonal expansion of cytotoxic T cells (T_C): secrete enzyme perforin to destroy infected cells.

Question 10

Outline the process of the humoral response

Answer 10

1. Complementary T_H lymphocytes bind to foreign antigen on antigen presenting T cells.
2. Release cytokines that stimulate clonal expansion (rapid mitosis) of complementary B lymphocytes.
3. B cells differentiaite into plasma cells
4. Plasma cells secrete antibodies with complementary variable region to antigen

Question 11

Give the long version of the cell-mediated response

Answer 11

1. Pathogen invades a host cell
2. The host cell displays the antigen on its major histocompatability complex (MHC) and becomes and antigen presenting cell (APC)
3. T killer cells with complementary receptor proteins bind to the APC
4. Cytokines secreted by active T Helper cells stimulate the T killer cell to divide by mitosis
5. T killer cell divides to form active T killer cells and T memory cells
6. Active T killer cells bind to the APCs and secrete chemical which cause pores to form in the cell membrane
7. The infected cell dies

Question 12

Give the long version of the humoral response, T helper activation

Answer 12

1. Bacterium is engulfed by a macrophage. Surface antigens are passed along the endoplasmic reticulum into a vesicle which are transported to the cell surface membrane
2. Macrophage acts as an APC and presents antigens on MHCs
3. Macrophage APC binds to T helper cell with complementary receptor proteins
4. The T helper cell is ‘activated’ and divides by mitosis to form T memory cells and active T helper cells

Question 13

Give the long version of the humoral response, effector stage

Answer 13

1. Antigens from APCs that are complementary to the antibodies on B cells bind and are taken in by endocytosis
2. The B cell acts as an APC and presents antigens on MHCs
3. An activated T helper cell (from the T helper activation stage) with a complementary receptor protein to the antigens binds to the APC. It produces cytokines.
4. Cytokines stimulate the B cell to divide by mitosis and from B memory cells and B effector cells
5. B effector cells differentiate into plasma cells
6. Plasma cells synthesise antibodies, effect of antibodies on another flashcard
7. T suppressor cells stop the immune response

Question 14

What is an antibody? Describe its structure.

Answer 14

Proteins secreted by plasma cells.

Quarternary structure: 2 ‘light chains’ held by disulfide bridges, 2 longer ‘heavy chains’

Binding sites on variable region of light chains have specific tertiary structure complementary to an antigen.

The rest of the molecule is known as the constant region.

Question 15

How do antibodies lead to the destruction of the pathogen?

Answer 15

• Formation of antigen-antibody complex results in agglutination. Causes microbes to clump together, makes phagocytosis easier.
• Activation of complement - lysis of the bacterial cell
• Opsonisation - antibodies coat microbes and mark them for phagocytosis
• Precipitation/ neutralisation (soluble toxins are made insoluble and inactive.

Question 16

What are memory cells?

Answer 16

Specialised T_H/ B cells produced from primary immune response.

Remain in blood at low levels.

Can divide very rapidly by mitosis if organism encounters the same pathogen again.

Question 17

Contrast the primary and secondary immune response

Answer 17

Secondary response:
* Faster rate of antibody production
* Shorter time lag between exposure and antibody production
* Higher concentration of antibodies
* Antibody level remains higher after the secondary response
* Pathogen usually destroyed before an symptoms

Question 18

Compare active and passive immunity

Answer 18

• Both involve antibodies
• Can both be natural or artificial

Question 19

Give examples of passive and active immunity

Answer 19

Passive natural:antibodies in breat milk/ across placenta

Passive artificial: anti-venom, needle stick injections

Active natural: humoral response to infection

Active artificial vaccination

Question 20

Contrast passive and active immunity

Answer 20

Passive:
* No memory cells and antibodies not replaced when broken down = short term
* Immediate
* Antibodies from external source
* Direct contact with antigen not necassary

Active:
* Memory cells produced = long-term
* Time lag
* Lymphocytes produce antibodies
* Direct contact with antigen necessary

Question 21

Explain the principles of vaccination

Answer 21

1. Vaccine contains dead/ inactive from of a pathogen or antigen
2. Triggers primary immune response
3. Memory cells are produced and remain in the bloodstream, so secondary response is rapid and produces higher concentration of antibodies
4. Pathogen is destoyed before it causes symptoms

Question 22

What is herd immunity?

Answer 22

Vaccinating 80-90% of population reduces available carriers of the pathogen to control disease transimission.

Protects individuals who have not been vaccinated e.g. those with a weak immune system.

Question 23

What is herd immunity?

Answer 23

Vaccinating 80-90% of population reduces available carriers of the pathogen to control disease transimission.

Protects individuals who have not been vaccinated e.g. those with a weak immune system.