UNIT 2: IMMUNE SYSTEMS

INTRODUCTION TO IMMUNITY

• An immune system is a system of

• biological structures and

• processes within an organism

• that protects against disease.

• In order to function properly, an immune system must detect a wide variety of agents, from viruses to parasitic worms, and distinguish them from the organism's own healthy tissue.

COUNTER ACTION OF THESE AGENTS

These agents are referred to as pathogens – an organism or substance that could cause a disease.

Pathogens can rapidly evolve and adapt to avoid detection and destruction by the immune system.

As a result, multiple defence mechanisms have also evolved to recognize and neutralize pathogens.

Physical barriers prevent pathogens such as bacteria and viruses from entering the organism.

If a pathogen breaches these barriers, the innate immune system provides an immediate, but non-specific response.

Innate immune systems are found in all plants and animals.

If pathogens successfully evade the innate response, vertebrates possess a second layer of protection, the acquired immune system,

which is activated by the innate response. Here, the immune system adapts its

response during an infection to improve its recognition of the pathogen.

This improved response is then retained after the pathogen has been eliminated, in the form of an immunological memory,

and allows the acquired immune system to mount faster and stronger attacks each time this pathogen is encountered.

Differences in the components of the immune system

Innate immune system Acquired immune system

Response is non-specific

Pathogen and antigen specific response

Exposure leads to immediate maximal response

Lag time between exposure and maximal response

Cell-mediated and humoral components

Cell-mediated and humoral components

No immunological memory

Exposure leads to immunological memory

Found in nearly all forms of life

Found only in jawed vertebrates

2. INNATE IMMUNITY

Innate immune responses are active immediately upon infection and are the same whether or not the pathogen has been encountered previously.

It includes barrier defensesand internal defenses

BARRIER DEFENSE SYSTEMIncludes:

the skinMucus membranes of digestive-, respiratory- urinary and reproductive

tractoBody secretions: mucus, saliva (lysozymes), tears, oil gland secretions, acid in stomach, sweat.

Mucus membranesSome cells in mucus membrane

produce mucus.Mucus is a viscous fluid that

enhances defences – trapping microbes and other foreign particles

In the trachea, ciliated epithelial cells sweep mucus and the trapped microbes upwards, helping to prevent infection of the lungs. mucus

Body secretions create an environment that is unfavourable for microbes.

Lysozymes in saliva, mucous secretions, and tears destroy susceptible bacteria as they enter the respiratory tract or openings around eyes.

Acid in stomach kill bacteriaOils and sweat give human skin a

pH between 3-5, which is acidic enough to prevent the growth of microorganisms.

INTERNAL DEFENSEIf the barrier defences are

damaged and pathogens do enter the body of an organism, a second line of defence will be activated.

This defence system is the internal defence system and is more sensitive and includes: phagocytosis and inflammation.

PHAGOCYTOSIS

process by which certain living cells called phagocytes ingest or engulf other cells or particles.

The phagocyte may be a one-celled organism, such as an amoeba, or one of the body cells, such as a leukocyte (white blood cell).

In higher animals phagocytosis is chiefly a defensive reaction against infection and invasion of the body

Different types of phagocytic cells:

Neutrophils engulf and destroy microbes

Macrophages are part of the lymphatic system and are found throughout the body

Eosinophils discharge destructive enzymes

Dendritic cells stimulate development of acquired immunity

NEUTROPHIL MACROPHAGE

EOSINOPHIL DENDRITIC CELL

PHAGOCYTE ENGULFING A BACTGERIUM CELL

PHAGOCYTOSIS OF AN AMOEBA

INFLAMMATORY RESPONSES

When injured or infected by pathogens, signalling molecules are released,

One example of a signalling molecules is histamine – stored in mast cells.

These molecules trigger the blood vessels to dilate and become more permeable

This increase local blood supply and allow more phagocytes and antimicrobial proteins to enter tissues

Pus, a fluid rich in white blood cells, dead microbes, and cell debris, accumulates at the site of inflammation

Fever is a systemic inflammatory response triggered by pyrogens released by macrophages, and toxins from pathogens

MAJOR EVENTS IN A LOCAL INFLAMMATORY RESPONSE

Activated macrophages and mast cells at the injury site release signalling molecules that act on nearby capillaries.

The capillaries dilate and become more permeable, allowing fluid containing antimibrobial peptides to enter the tissue. Signaling molecules released by immune cells attract additional phagocytic cells.

Phagocytic cells digest pathogens and cell debris at the site, and the tissue heals

ACQUIRED IMMUNITY

Acquired immunity is a specific immune response system through which the body specifically detects and destroys particular substances.

This immunity protects us against infectious diseases as the body has learnt to recognize foreign substances.

It produces a specific reaction to each infectious agent, eradicating that agent from the body.

This ability to recognize a pathogen that has previously elicited an immune response is the basis for acquiring immunity to specific diseases.

Hence, we suffer from many diseases, such as chicken pox, measles etc. only once.

Thus we can summarize: Acquired immunity involves 2 main activities:Destruction of the invadersMemory of this response

WHICH CELLS ARE INVOLVED IN ACQUIRED IMMUNITY RESPONSE?

Special leucocytes, called the lymphocytes are released from the bone marrow.

Some reach the Thymus gland and mature to form T-lymphocytes (T-cells)

Some become B-lymphocytes (B-cells) and is present in the bone marrow and lymph nodes)

ACTIVE ACQUIRED IMMUNITYActive immunity occurs when a

person has already been exposed to antigens (from pathogens)

And develops a secondary response against specific pathogens.

People receive vaccinations to develop a primary response, and then if they get the pathogens later, active immunity helps to fight them off.

Antigens are foreign molecules, found on the surface of pathogens, each pathogen has a specific antigen.

The immunity system has countless amounts of B-Lymphocytes.

Each B-lymphocyte is able to recognize a specific antigen.

The B-lymphocytes then produce antibodies that will bond to the antigens.

This will neutralize or destroy the pathogen.

HOW B-LYMPHOCYTES WORK

antibody

antibodies

oThe B-lymphocyte form antibodies

o One type of B-lymphocyte is activated by a specific antigen on the surface of a foreign body

The antibodies bond to the antigens and destroy the foreign body.

PASSIVE IMMUNITYIn natural passive immunity, antibodies are

passed from a mother to a child. Antibodies can be transferred through the

placenta, or transmitted through the colostrum. The antibodies transmitted through the

colostrum and placenta generally only last for several weeks, which is long enough to allow the baby to start to build up its own immune system and to make its own antibodies.

Artificial passive immunity involves the introduction of antibodies through means such as injection - VACCINATIONS.

HUMORAL VS. CELL MEDIATED IMMUNITY

Humoral immunity - deals with infectious agents in the blood and body tissues

Cell-mediated immunity - deals with body cells that have been infected.

In general, the humoral system is managed by B-cells (with help from T-cells).

The cell-mediated system is managed by T-cells.

VACCINATIONSAll vaccinations work by presenting

a foreign antigen to the immune system in order to evoke an immune response, but there are several ways to do this.

We will look at 4 methods:

1. Using an inactivated vaccine

An inactivated vaccine consists of virus or bacteria that are grown in culture and then killed .

Although the virus or bacteria particles are destroyed and cannot replicate, the virus capsid proteins or bacterial wall are intact enough to be recognized and remembered by the immune system.

This evokes an immune response.

2. Using an attenuated vaccine

In an attenuated vaccine, live virus or bacteria with very low virulence are administered.

They will replicate, but locally or very slowly.

Which causes an immune response to produce antibodies.

3. Virus-like particle vaccinesVirus-like particle vaccines consist of viral

protein(s) derived from the structural proteins of a virus.

These proteins can self-assemble into particles that resemble the virus from which they were derived but lack viral nucleic acid, meaning that they are not infectious.

The human papillomavirus and Hepatitis B virus vaccines are two virus-like particle-based vaccines currently in clinical use.

4. A subunit vaccine

A subunit vaccine presents an antigen to the immune system without introducing viral particles.

One method of production involves isolation of a specific protein from a virus or bacterium and administering this by itself.

ANTIBIOTICS

Antibiotics are also known as anti-bacterials.

They are drugs used to treat infections caused by bacteria.

The first antibiotic was penicillin.

How do antibiotics work?

Although there are a number of different types of antibiotic they all work in one of two ways: A bactericidal antibiotic kills the

bacteria. Penicillin is a bactericidal. A bactericidal usually either interferes with the formation of the bacterium's cell wall or its cell contents.

A bacteriostatic stops bacteria from multiplying.

If antibiotics are overused or used incorrectly there is a chance that the bacteria will become resistant - the antibiotic becomes less effective against that type of bacterium.

REFERENCES

Campbell and Reece.2010.Biology.8th ed.Cape Town:Pearson. Chapt 43J. Williamson and E. Pretorius.2014.Life Sciences Learning Guide.Johannesburg:University of Johannesburg