infections and immunity

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• Pathogens

• Microorganisms that cause diseases

• Commensals

• Those that live in harmony with the host without causing any damage to it


INFECTION

• Lodging and multiplication of pathogens in or on tissues of a host


Primary infection

• Infection that develops in an otherwise healthy host


Re - infection

• Infection by the same organism again in the same host


Secondary infection

• Infection in the host affected by primary infection by a new organism


Disease-causing organisms

Organisms that cause disease are called pathogens.

bacteria fungi

protozoa

virus

What are the four major types of pathogen?


How do pathogens cause illness?

ToxinsToxins are harmful substances produced by the pathogen that poison the body’s tissue and enzymes.

ReproductionA rise in the number of pathogens can damage a cell, even causing it to burst. Some pathogens hijack resources that the cell needs to survive.

Immune responseSites of infection often become swollen, sore and hot as a result of increased blood flow.

Pathogens cause illness in three main ways:


How are pathogens spread?

Different pathogens have different transmission routes:

direct contact indirect contact

food and water airborne droplets

insect bites

Immunity to Infection


Fighting infection

How does the body fight infection?

Immunity to Infection

• Immunity is the acquired ability to defend against infection by disease-causing organisms.

Overview of your immune system

• First line of defense: Physical barriers that viruses, bacteria must cross– skin covers ~2 m2

– Mucous membranes that line digestive, respiratory, reproductive tracts cover ~400 m2

• Second line of defense: Innate immune system (germline-encoded receptors -- no adaptation to specific pathogens)– Macrophages (Greek for “big eater”), neutrophils, natural killer (NK) cells

• Third line of defense (vertebrates only): Adaptive immune system (adapts to defend against specific pathogens using variable receptors)– B cells make antibodies that vary -- can make an antibody specific for any

new antigen– T cells mediate cellular responses using variable receptors (T cell receptors;

TCRs)


First line of defence


Painkillers can relieve the symptoms of an infection but do not kill the pathogen.

Invasion!

A pathogen enters your body through direct or indirect contact. What happens next?

Your immune system begins to mount an attack.

The pathogen begins toreproduce and make toxins that destroythe body’s cells and make you feel unwell.


Second line of defence – Innate immunityMacrophages


Second line of defence - Natural killer (NK) cells

• Can kill tumor cells, virus-infected cells, bacteria, parasites, fungi in tissues

• Identify targets based on “missing self” – Two types of NK receptors: inhibitory and activating

– If inhibitory receptor recognizes a self protein (a class I MHC molecule) on a target cell, the NK cell is turned OFF even if activating receptor binds a ligand on the same target cell

– If activating receptor binds a ligand, but inhibitory receptor does not (target cell has down-regulated class I MHC proteins), NK cells kill

– Many virally-infected cells and tumor cells down-regulate expression of class I MHC molecules (NK cells important for preventing cancers)


Resisting attack

TB bacteria have a number of adaptations that enable them to resist phagocytosis:

They produce chemicals that prevent macrophages from being attracted to sites of infection.

They have a waxy cell wall that resists enzyme attack and secretes chemicals that block lysosomes from fusing with phagosomes.

How does the immune system deal with these bacteria?


Third line of defence - Lymphocytes

Lymphocytes are a type of white blood cell found in the blood or lymph nodes and made by bone marrow. There are several types of lymphocyte, including:

T-lymphocytes – recognise antigens on pathogens and either attack them directly or co-ordinate the activity of other cells of the immune system.

B-lymphocytes – recognise antigens and produce special chemicals called antibodies.


B lymphocytes

Antibodies are specialY-shaped proteins produced byB-lymphocytes in response to antigens.

Antibodies work by binding to antigens on pathogens, ‘labelling’ them and causing them to clump together. The pathogen can then be destroyed by:

the antibodies themselves.

phagocytosis by macrophages

T-lymphocytes


Antibodies

Each different type of antigen causes a different type of antibody to be produced.

An antibody can only bind to the antigen that caused it to be produced.


Delayed response

The B-lymphocyte that produces the correct antibody for the antigen begins dividing to produce many more antibody-producing cells.

It takes a few days to produce enough antibodies to destroy the pathogen. This means there is delay between infection and the person beginning to feel better.

Once a pathogen has been destroyed, a few memory cells remain. These recognize the pathogen if it re-infects, and make the immune response much quicker and more effective. This is called active immunity.


Antibody levels during infection


T cells

How does the body deal with pathogens that are inside cells?

Viruses and bacteria that infect cells leave antigens on the surface of the cell they infect.

T-lymphocytes recognize these antigens by receptors on their surface and destroy the whole infected cell.

infected cellT-cell

antigen receptor


Types of Immunity

Artificially acquired

Passive immunityActive immunity

Naturally acquired Naturally acquired

Artificially acquired


Types of Immunity

• Natural immunity is the result of a body’s previous encounter with an organism.

• Artificial immunity results from the injection of a vaccine or an antibody. Vaccines stimulate active immunity whereas injection antibody or antiserum is an example of passive immunity.


Types of Immunity

• Active immunity is when the immune system encounters and antigen and is primed to recognise it and destroy it quickly the next time it is encountered. This is active immunity because the body’s immune system prepares itself for future challenges.

• Passive immunity is short-term and involves the transfer of immunity from one individual to another via antibody-rich serum. This may be artificial as is the case with anti-venom or natural, as in antibodies crossing the placenta to protect the developing foetus.


Immunity

Active immunityProduction of a person’s own

antibodies. Long lasting

Passive immunityAn individual is given antibodies by

another Short-term resistance (weeks- 6months)

Natural ActiveWhen pathogen

enters body in the normal way, we make antibodies

Natural PassiveBaby in utero

(placenta)Breast-fed babies

Artificial PassiveGamma globulin

injectionExtremely fast, but

short lived (e.g. snake venom)

Edward Jenner

Artificial ActiveVaccination –

usually contains a safe antigen from

the pathogen. Person makes antibodies

without becoming ill


Passive immunity

Many snakes produce a powerful nerve toxin that can be lethal to humans.

Antivenin contains antibodies to give instant immunity. It is produced by injecting horses with small, non-lethal doses of venom. Over time, the horses produce antibodies, which are extracted and processed.

People bitten by poisonous snakes can be treated with antivenin.

Because the person didn’t make the antibodies themselves,this is called passive immunity.


Vaccines contain a small amount of dead or weakened pathogen particles.

What are vaccines?

A vaccine stimulates the production of antibodies and memory cells against the target pathogen, without making the person ill.

If a vaccinated person is later infected by the same pathogen, their immune system can destroy it very quickly.

Parents of two-year-old children are offered a combined measles, mumps and rubella (MMR) vaccine to protect their child. What has happened to MMR vaccination rates recently?


Measles, mumps and rubella

MMR vaccination rates used to be high, but fell following a media scare story.

The media reported on controversial research speculating that MMR could cause autism, a behavioural disorder causing learning and communication difficulties.


Glossary (1/2)

antibiotic – A drug that can destroy or prevent the growth of bacteria.

antibody – A Y-shaped protein produced by the body that binds to antigens.

antigen – A substance on pathogens that stimulates the production of antibodies.

B-lymphocyte – A white blood cell that produces antibodies.

immunization – The process of protecting against infection by using a vaccine.

immune response – The body’s defence against foreign material such as pathogens.


Glossary (2/2)

immunity – The ability to fight infection by pathogens. It can be active or passive.

pathogen – A disease-causing micro-organism.

phagocytosis – The process where a type of white blood cell called a macrophage ingests and destroys a pathogen.

T-lymphocyte – A type of white blood cell that recognises and destroys pathogens, and co-ordinates the immune response.

tuberculosis – A serious bacterial disease that mainly affects the respiratory system.

vaccine – A small amount of dead or weakened pathogen that stimulates antibody production.


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infections and immunity

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