2.4: Cell Recognition and the Immune System

What can antigens help the body to identify?

Pathogens, cells from other organisms of the same species, abnormal body cells (e.g. cancer), and toxins

What is an antigen?

A protein on the surface of a pathogen that stimulates an immune response

What is a pathogen?

A disease-causing microorganism (e.g bacteria, birus, fungi)

How do bacteria cause disease?

By producing toxins

How do viruses cause disease?

By injecting their nucleic acids into cells, causing them to divide and burst.

What defenses does the body have against pathogens?

1. Barriers: preventing pathogens from entering the body (e.g. skin)

2. Phagocytes: white blood cells that carry out phagocytosis and stimulate specific response

3. Specific response: uses lymphocytes to produce memory cells and antibodies

What are the barriers?

Skin: an impermeable layer made of keratin

Cillia and mucus in lungs: captures and traps pathogens

Stomach acid: denatures/breaks down pathogens

Describe the process of phagocytosis

Pathogen releases chemicals which attracts phagocyte

Phagocyte binds to pathogen

Phagocyte engulfs pathogen anf forms a phagosome around the pathogen

Lysosomes inside the pahgocyte release digestive enzymes into the phagosome, breaking down the the pathogen by hydrolysis

Describe specific response

Phagocytes perform phagocytosis (engulf and destory pathogen) without destroying the antigen, they place it on their surface and present the antigens

T-cells bind to the antigen and become stimulated, dividing by mitosis to form 3 types of cells (t helper cells, cytotoxic t cells, and t memory cells.

What do t helper cells do?

Stimulate b-lymphocytes:

B lymphocytes (b cells) become stimulated and divide by mitosis to make 2 types of cells (plasma cells and b memory cells)

Plasma cells make antibodies

Memory cells provide long term immunity.

What do t memory cells do?

Provide long-term immunity

How does the immune response lead to production of antibodies?

The phagocytes stimulate the t cells, the t cells from t helper cells

t helper cells stimulate the b cells which form plasma cells

the plasma cells make antibodies.

What is an antibody?

A globular protein made by plasma cells

Has 3 regions: variable region, hinge region, and a constant region.

Variable region has a different shape in each antibody, contains antigen binding sites. These bind to complimentary antigens on a pathogen to form an antigen-antibody complex, destroying the pathogen.

Hinge region gives the antibody flexibility

Constant region is the same shape in all antibodies, binds to phagocytes to help with phagocytosis

How do memory cells work?

Made during the specific immune response after a new infection by a pathogen (known as a primary infection)

B/T cells remain in the blood

If a person is reinfected by the same pathogen (known as a secondary infection), the memory cells will recognise the pathogen and produce antibodies rapidly and to a large amount

Therefore the pathogen is killed before it can cause harm

How does a vaccine produce immunity?

Involves giving an injection that contains dead/weakened/inactive pathogens that carry antigens which stimulates the immune response leading to production of antibodies and memory cells

Active vs passive immunity

Active = individual has memory cells - can make their own antibodies and provides long term immunity

Passive = person given antibodies, these work then die, no long term immunity, no memory cells

How does active immunity occur?

Naturally: by primary infection

Artifically: by vaccination

How does passive immunity occur?

Naturally: from mother to baby (placenta/breast milk)

Artifically: by injection

What is herd immunity?

When a large proportion of the population is vaccinated, therfore most people will be immune. Only a few will not be immune, increases chance of non-immune person coming into contact with an immune person, so the pathogen has nowhere to go and dies out.

What are problems with vaccine programs?

New strains of pathogen could emerge (why people still get ill with colds every year - new strains always emerging)

Ethics/personal objections (can’t force people to get vaccinated, their body, thier choice or the use of animal testing)

Not everyone can be vaccinated (e.g. immunocompromised persons)

Describe the cell-mediated response

An antigen-presenting cell arises as a result of phagocytosis. T-cells have receptors on their surface much like the phagocyte. The receptor binds to the antigens on the cell.

These t-cells can differentiate into helper t-cells, cytotoxic t-cells, or t-cells which simulate phagocytosis

What is the role of cytotoxic t-cells?

Produce a protein called perforin which permeates the cell membrane of pathogens this allow substances to enter the cell and eventually results in cell death