Topic 2: Immunity Flashcards by Tamilselvi Sivakaran

What are cells capable of recognising?

-Pathogens
-Cells from other organisms
-Abnormal cells
-Toxins

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Define the term antigen

A protein found on the cell surface membrane that triggers an immune response when it identifies a non-self cell

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What is antigen variability?

When the antigens present on the pathogen’s surface, changes frequently due to genetic mutations.

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Why is antigen variability a problem?

Lymphocytes and memory cells produce a specific immune response.
- The surface receptors on the lymphocytes and memory cells are complimentary to only 1 antigen.
- When the antigen on the pathogen changes shape, the lymphocytes and memory cells can no longer bind
- No secondary immune response

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What are phagocytes and where are they produced?

Type of white blood cell produced in the bone marrow.

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What are the 2 types of phagocytes?

-Macrophages
-Neutrophils

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What do macrophages do?

They stick to proteins on the outside of the bacteria to make them more appealing for the neutrophils to destroy them.

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What does neutrophils do?

Engulfs and digests pathogens.

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What is phagocytosis?

A non specific immune response where phagocytes detects and engulfs pathogens

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Describe the process of phagocytosis

-The chemicals released by the pathogen attracts the phagocytes.
- The phagocytes have receptors that recognises the antigens on the pathogen as a non self cell
- Phagocyte attaches to the pathogen by binding to the complementary antigen
- Pathogen is engulfed by endocytosis forming a phagosome
-Lysosome fuses with the phagosome to form phagolysosome
- Lysosomes releases a digestive enzyme called lysozymes which digests the pathogen

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What are lymphocytes?

A type of white blood cell involved in the specific immune response.

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Where are lymphocytes made?

In the bone marrow
B cells mature here but T cells mature in the Thymus

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What is Antigen Present Cell (APC) ?

Any cell that presents a non self antigen on its surface

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What are the types of APC’s?

-Infected body cells
-macrophages
-cells from a transplanted organ
-Cancer cells

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What are T cell responses described as cell mediated?

Because T-Cells only respond to antigens which are presented on APC

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What are the steps of cell mediated response?

Once pathogen has been engulfed and destroyed by a phagocyte the antigen is presented on the cell surface
Helper T cells have receptors on the surface which can bind to the antigen on the APC
This activates helper T cells to divide by mitosis to replicate and make clones.
Cloned helper T cells can differentiate into other cells

What happens to cloned helper T cells?

remain as helper T cells to active B lymphocytes
stimulate macrophages for more phagocytosis
become memory cells for that specific antigen
become Cytotoxic T cells

What is the role of Cytotoxic T cells?

Destroy abnormal or infected cells by releasing perforin (a protein), which embeds in the cell surface membrane, making a pore so that substances can leave or enter the cell, causing cell death.

What happens when an antigen comes into contact with a B cell?

The antigen collides with the complementary antibody on the B cell. It takes it in by endocytosis and then presents it on its cell surface membrane.

What happens when a B cell collides with a helper T cell?

Activates the B cell to undergo clonal selection and clonal expansion.

What happens to B cells after they undergo mitosis?

Differentiate into memory and plasma cells.

What does plasma cells do?

Make and releases antibodies

What’s the role of B memory cells?

They can divide rapidly into plasma cells when re infected with the same pathogen to make large number of antibodies rapidly.

(Known as active immunity)

Describe the structure of an antibody

Has a quaternary structure.
-2 heavy polypeptide chains bonded to 2 light chains by disulphide bonds
-Each polypeptide chain has a constant and variable region

What is the importance of the variable region?

It’s where the antibody attaches to the antigen to form antigen-antibody complex as there’s an antigen binding site at the ends of the variable region.

Describe the binding site of the antibody

Amino acid sequence in the variable region are different for each antibody. Giving its specificity for binding to antigens. Sites are specific to the epitope.

Define the term epitope

Part of the antigen that binds to the antibody

Where is the hinge region formed?

Found where the disulphide bonds joins the heavy chains

How is the hinge region useful for the antibody?

Gives flexibility which allows antigen binding site to be placed at different angles when binding to antigens.

What is agglutination?

As the antibody has at least 2 antigen binding sites, they can bind to more than 1 virus or bacterium at the same time, causing groups of the same pathogen to clump together.

What is the effect of the antibody binding to the antigen?

Neutralises the pathogen or acts like a marker to attract phagocytes to engulf and destroy the pathogen.

How does agglutination make phagocytosis easier?

Phagocytes can phagocytose more pathogens at the same time due to them being clumped together.

What happens in the humoral response?

Receptors on naive B cell bind to complementary antigens on a pathogen. The B cell then internalises the pathogen and presents the pathogens antigens on its surface. The processed antigens binds to helper T cells, which stimulate the B cell to clone. The cloned B cells differentiate into Plasma cells which secrete proteins called antibodies, and memory cells, which remain in the body after the pathogen is gone. If body re counters the same antigen the memory cells clone themselves and differentiate once again

Describe how antibodies lead to destruction of a pathogen

Antibodies bind to complementary antigens on the surface of the pathogen, forming Antibody-Antigen complexes. Causes pathogens to clump together in a process called agglutination, making it easier for phagocytes to engulf and destroy pathogens.

Describe the cell - mediated immune response.

A naive helper T cells receptor binds to an antigen on an antigen presenting cell, stimulating the T cell to clone itself. Cloned helper T cells stimulates cytotoxic T cells to target virus-infected body cells When cytotoxic T cell receptors binds to antigens in virus infected body cells, they release perforin which creates pores on the cell surface membrane allowing substances to enter and leave the cell causing cell death.

What can cloned T helper cells stimulate?

- B cells to clone -Phagocytes to undergo phagocytosis - Cytotoxic T cells to target virus-infected body cells.

Describe the different ways that specific immune response leads to the destruction of antigens.

- B cells clones differentiate into plasma cells which secretes antibodies. Antibodies binds to antigens on pathogens causing them to agglutinate, making it easier for phagocytes to phagocytose. - Helper T cell clones stimulates phagocytes to destroy antigens by phagocytosis. - Helper T cells stimulate cytotoxic T cells to secrete perforin which causes cell death.

Explain why it’s important that there’s lots of diversity between lymphocytes

Body needs lymphocytes with a diverse range of receptors, which can each recognise a different antigen. Allows body to respond to infection by a wide variety of pathogens.