A-Level Biology AQA Notes

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Components of the Immune System

Antigens are any part of an organism/substance which is recognised as foreign by the immune system and goes on to trigger an immune response.

Antibodies are a protein produced by lymphocytes in response to the presence of the corresponding antigen.

Antibodies agglutinate pathogens by forming antigen-antibody complexes, leading to phagocytosis & neutralise toxins.

Cell-mediated Immunity

Antigen from the pathogen is displayed on the cell surface of body cells or phagocytes after phagocytosis
T cells with the correct specific receptor bind with the antigen and are activated
They divide by mitosis (clonal expansion) and differentiate into T helper, cytotoxic and memory cells.

Humoral Immunity

The humoral response is best at fighting pathogens which are free in the bodily fluids

Free antigen binds to a complementary B cell receptor, activating the B cell (clonal selection)
The pathogen is endocytosed, and the antigen presented on the plasma membrane
T helper cell binds to the presented antigen and stimulates the B cell to divide by mitosis (clonal expansion)
The B cell differentiates to plasma and memory cells

Primary & Secondary Immune Response

The primary immune response is when a pathogen infects the body for the first time the initial immune response is slow
The secondary immune response is a more rapid and vigorous response caused by a second or subsequent infection by the same pathogens. This is due to the presence of memory cells.

Vaccination

Vaccination is the introduction into the body of a vaccine containing disease antigens, by injection or mouth, in order to induce artificial immunity
Vaccines work by injecting weakened/dead pathogens into the body to stimulate an immune response, to form memory cells against the specific antigen, which destroy the pathogen quickly upon infection.
Herd immunity is when the vaccination of a significant proportion of the population provides protection for individuals who have not developed immunity
Pathogen may mutate so that its antigens change suddenly (antigenic variability) So the vaccine is now ineffective to the new antigens.
Ethical considerations: side effects, financial cost, right to choose, animal testing of vaccines, human trials
Active immunity occurs when specific antibodies are produced by the individual’s own immune system
Passive immunity occurs when specific antibodies are introduced to the individual from an outside source.

Immunity	Example
Natural Active	Direct contact with pathogen
Natural Passive	Antibodies through breastmilk
Artificial Active	Vaccination
Artificial Passive	Injection of antibodies

Human Immunodeficiency Virus (HIV)

HIV replicates in T helper cells, causing the symptoms of AIDs due the to decreased cell count. The compromised immune system leads to the risk of serious infections

Antibiotics kill bacteria by targeting bacteria specific enzymes or organelles. They are ineffective against viruses due to the virus using the host’s machinery.

Using Monoclonal Antibodies

Drugs can be attached to monoclonal antibodies, in order to ensure the delivery of the drug to specific cell types e.g. cytotoxic drug to a cancer cell
Disease diagnosis can occur by testing for the presence of specific pathogen antibodies in the blood.
Monoclonal antibodies are also used for pregnancy testing
Measurement & diagnosis of antigen occur in the ELISA test where different monoclonal antibodies are bound to the surface of a well. They attach to antigen present in a sample, allowing the attachment of a detection antibody. An enzyme attached to the detection antibody digests a substrate, which is added, causing a colour change. The colour intensity corresponds to the amount of the antigen present in the sample
Ethical considerations: treatment may cause death (risky), use of animals for production may cause harm, human trials

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