Cell Recognition and the Immune System

Cell Recognition and the Immune System - AQA A-Level Biology

What is an antigen?

A foreign surface protein that stimulates an immune response

What do antigens help our immune systems identify?

Our own cells

Pathogens

Cells from others of the same species

Cancer / abnormal cells

Toxins

What feature of antigens allows them to be identified and detected?

They have specific shapes due to their tertiary structure

What are the two types of defence mechanisms?

Specific and non-specific

What is a non-specific defence mechanism?

A response that works on all pathogens - regardless of their antigens

What is a specific defence mechanism?

A response that directly targets a specific pathogen (due to its antigen)

What are the two types of non-specific defence mechanisms?

Physical barriers

Phagocytosis

What are the two types of specific defence mechanisms?

Cell-mediated response

Humoral response

How does the skin act as a physical barrier?

It is an impermeable barrier that pathogens can’t penetrate

How does epithelial mucus act as a physical barrier?

Pathogens stick to the mucus so are immobilised

How do cilia act as a physical barrier?

Move dirt and pathogens back up from the lungs

How does stomach acid act as a physical barrier?

HCl denatures the enzymes / coat proteins, so pathogens are destroyed

What is the process of phagocytosis?

1. Phagocyte/macrophage detects and moves toward the pathogen

2. It binds to the antigens on the pathogen and engulfs it into a vesicle

3. The vesicle fuse with the phagosome, releasing the pathogen

4. Lysosomes also fuse with the phagosome, releasing lysozymes which hydrolyse the pathogen

5. Hydrolysed products are absorbed by the phagocyte

6. Antigenic molecules are presented on the surface of the cell, so the phagocyte becomes an antigen presenting cell

What does a phagocyte become after phagocytosis?

Antigen presenting cell

What are lymphocytes?

A type of white blood cell involved in specific response

What are 2 types of lymphocytes?

B-cells

T-cells

Where are B-cells made?

Bone marrow

Where are T-cells made?

Bone marrow

Where are all lymphocytes made?

Bone marow

Where do B-cells mature?

Bone marrow?

Where do T-cells mature?

Thymus

What type of immunity are B-cells involved in?

Humoral immunity

What type of immunity are T-cells involved in?

Cellular immunity

What are the 2 types of T-cell you need to know for this spec?

Helper T cells (T_H cells)

Cytotoxic T cells (T_C cells)

How do T-cells help in specific responses?

They each have receptors that are specific to an antigen on a foreign cell

What is it called when a T-cell binds to a foreign cell’s antigens

Antigen-receptor complex

Instead of a foreign cell, what else might a T-cell bind to?

The antigens on antigen presenting cells that have engulfed the foreign cell

What protein do T_C cells produce?

Perforin

What does perforin do?

Makes holes in the cell surface membrane, making it permeable

How do T_C cells cause cell death?

They produce perforin, which makes holes in the cell surface membrane, making it permeable

What is clonal selection?

When receptors on T_H cells bind to the antigens they are specific to

In what type of response does clonal selection occur?

Both cell-mediated/cellular and humoral response

What happens when a T_H cell binds to the antigen it’s specific to?

The T_H cell is activated

What happens when a T_H cell is activated?

It divides by mitosis to produce T_H cell clones (this is clonal expansion)

Release cytokines to activate T_C cells

Activate B-cells (through clonal selection)

Stimulate phagocytosis

What occurs during cell-mediated / cellular response?

1. Clonal selection: receptors on T_H cells bind to the antigens they are specific to

2. This activates the T_H cells so they

   • Divide by mitosis to produce T_H cell clones (clonal expansion)

   • Release cytokines to activate T_C cells

   • Activate B cells (clonal selection)

   • Stimulate phagocytosis

How are B-cells specific to antigens?

They have antibodies on their surface which are specific to certain antigens