immune system

antigen

foreign protein/molecule that stimulates an immune response

Give 2 ways in which pathogens can cause disease

1.release toxins

2.kill cells

4 things antigens are used to identify

1. pathogens

2. toxins

3. abnormal body cells

4. cells from organisms of the same species

Suggest one reason why very few live births result from an embryo that is implanted into the womb

• embryo has foreign antigen

• embryos rejected by immune system

if physical barriers fail, what is the next line of defence in an organism?

phagocytosis

antibodies

a protein specific to an antigen produced by B cells

explain the structure of an antibody

1. 4 polypeptide chains

   • 2 heavy chains and 2 light chains

2. variable region

   • contains specific binding site

3. generic constant region

   • allows attachment to phagocytic cells

4. hinge

   • allows for flexibility

   • hence can bind to multiple antigens

antibody diagram

antigen-antibody complex

what is the role of the disulfide bridge in forming the quaternary structure of an antibody

joins 2 polypeptides

explain why antibodies are only effective against a specific pathogen

• antigens have a specific tertiary 3D structure

• shape of the antibody is complementary to the antigen

• antibody binds to the antigen, forming an antibody-antigen complex

state how antibodies deal with infections via neutralisation

antibodies bind to toxins and prevents the binding of these toxins to the host cells

monoclonal antibody

an antibody produced from identical B cells

give one example of using monoclonal antibodies in medical treatment

carries medicine to specific cells

describe 3 ethical considerations in the use of monoclonal antibodies

1. animal testing

- involves use of mice in antibody production

2. informed consent

- patients must know all the benefits + risks of the drugs

3. drug trials

- testing on volunteers can be dangerous

- there can be issues over trial conduct

positive result in ELISA test

1. The first antibody binds to complementary antigen

2. A second antibody with enzyme attached is added

3. The second antibody attaches to the first antibody

4. Solution containing substrate is added and colour changes

phagocytosis

1. A phagocyte recognises a foreign antigen on a pathogen and moves towards pathogen via chemotaxis

2. pathogen is engulfed (via endocytosis) and enclosed in a phagosome

3. vacuole/phagosome fuses with lysosome, forming phagolysosome

4. lysosome contains enzyme called lysozyme

5. pathogen destroyed by lysozymes

6. phagocyte absorbs the products from pathogen hydrolysis

7. phagocyte presents the pathogens antigens

   • antigens from pathogens are displayed on cell surface membrane

in what case is an immune response disadvantageous

in organ transplants:

- immune system recognises the organ as 'non-self' and attempts to destroy it

how do doctors minimise the risk of organ rejection?

1. tissue type is matched

2. immunosuppressants are used

outline the process of the cell-mediated/cellular immune response?

1. complementary helper T cells bind to foreign antigen on antigen-presenting cell

2. releases cytokines

3. Helper T cells divide by mitosis (clonal expansion of complementary T helper cells)

- become memory cells or trigger humoral response

4. clonal expansion of cytotoxic T cells (TC)

How do Cytoxic T cells kill infected cells?

by producing an enzyme called perforin that makes holes in the cell surface membrane

describe the steps in the humoral response

1. B cells bind to complementary antigen

2. B cells become activated and divide via mitosis to produce plasma cells (clonal expansion of B cells)

3. plasma cells produce and secrete specific antibodies that are complementary/specific to the antigen

4. some B cells develop into memory cells

- circulate the blood

- secondary response

in what cases may reinfection of the same pathogen occur?

1. different strains of the same pathogen

2. antigenic variability (different antigens)

3. when memory cells arent useful and produce an incorrectly shaped antibody

what is antigenic variation and what are its consequences?

When pathogens change their surface antigens

• The immune system cannot recognise this new antigen, so the memory cells don’t recognise it

• So a primary response must occur, which takes time and gives the individual symptoms

what are 2 issues with taking a vaccine orally?

1. there are enzymes in the gut that may break down the oral tablet

2. the molecules of the vaccine could be too large to be absorbed into the gut

Explain why antibiotics are ineffective against viruses

1. antibiotics work by preventing bacteria from making normal cell walls

   • but viruses rely on host cells for metabolic activities

2. viruses have a protein coat

   • so they dont have sites where the antibiotics can work

Describe how vaccines can lead to the production of antibodies against a disease-causing organism.

1. Vaccine contains antigen from pathogen

2. Macrophage presents antigen on its surface

3. Helper T cell with complementary receptor protein binds to antigen

4. Helper T cell stimulates B cell

5. B cell secrets large amounts of antibody

Why may vaccination not eliminate a disease?

1. fails to induce immunity in some people

2. vaccinated people may harbour the pathogen and infect others

3. antigenic variability (where antigens change frequently)

4. varieties of pathogen/strains

5. some pathogens 'hide' from the bodys immune system

6. medical/religious/ethical objections to taking the vaccine

what is herd immunity?

When a sufficiently large proportion/ majority of the population has been vaccinated to make it difficult for a pathogen to spread within that population

Why is herd immunity important?

• prevents transmission of pathogen in community

• Reduced infection rate in population

• Prevents future pandemics

• Protects vulnerable individuals e.g. babies

• its impossible to vaccinate everyone due to religious/medical/ethical objections

Describe the difference between active and passive immunity.

1. Active involves memory cells, passive does not

2. Active involves production of antibody by plasma cells / memory cells

3. Passive involves antibody introduced into body from outside

4. Active long term, because antibody produced in response to antigen

5. Passive short term, because antibody is broken down

6. Active takes time to work, passive fast acting

Describe the structure of HIV

1. lipid envelope + attachment proteins embedded

2. Capsid encloses 2 single RNA strands + enzymes including reverse transcriptase

3. Its a retrovirus bc it contains RNA

Describe how HIV is replicated

1. Attachment proteins attach to receptors on helper T cell

2. RNA enters cell

3. Reverse transcriptase converts RNA to DNA

4. Viral protein/capsid/enzymes produced

5. DNA transcribed to RNA then translated into new viral particles

6. Virus particles are assembled and released from the cell

how does HIV infect a host?

infects and kills helper T cells

- their role in the immune response is stopped

Explain how HIV affected the production of antibodies when AIDS develops in a person

No antibodies produced b/c HIV destroys helper T cells

- so no B cells are stimulated