Describe the three lines of body defence in humans: (i) first line of defence - physical - chemical (ii) second line of defence - fever - inflammation - phagocytosis (iii) third line of defence - antibody

Body Defence

11.1

Body Defence

Introduction

The body defence system reacts when pathogen infects the body.
A pathogen is a microorganism that causes diseases.
Examples of pathogens include bacteria (singular:bacterium), viruses and parasites.
Pathogens can only cause diseases if they successfully infect body cells.
The body defence system recognises pathogens as foreign particles that enter the body and subsequently stimulate the immune response.
Antigen stimulate the lymphocytes to produce antibodies into the blood flow to destroy antigens.
Antibodies are proteins found in the lymphocytes surface or proteins released by lymphocytes into the blood plasma.
The interaction berween antibodies and antigens that cause antigens to be destroyed is called the immune response.
Immunity is the body's ability to fight infections caused by pathogens or other foreign objects, through specific attacks on the pathogens.
When our body can fight a disease, we are said to be immune to that disease.
There are three lines of defence to fight against diseases in our body.
- The first line of defence.
- The second line of defence.
- The third line of defence.

The First Line of Defence

Consists of the physical and chemical line-up that prevent pathogens from entering the body.
The first line defence is the defence line that is not specific and acts to prevent pathogens from entering the body.

Lysozyme

Lysozyme found in the secretion of tears, nasal mucus and saliva, is an antimicrobial protein that can dissolve and destroy some types of bacteria.

Mucous Membrane

The mucous membrane that lines the respiratory tract secretes a sticky fluid called mucus.
Mucus contains lysozyme that destroys bacteria found in the air that enters the respiratory system.

Mechanism of Blood Clotting

The blood clotting mechanism prevents bacteria from entering through wounds.

Hydrochloric Acid

Hydrochloric acid in the stomach destroys bacteria present in food and drinks.

Skin

It is the physical defence that prevents the entry of pathogens because it is tough and is difficult for microorganisms to penetrate.
The continuous shedding of dead cell layers on the skin surface makes it difficult for various types of microorganism to grow.
Acts as a chemical shield through the secretion of sebum.
Oil and acid in the sebum prevent the growth of various types of microorganisms.
Sweat secreted by the skin contains lysozyme, a type of enzyme that breaks down the cell wall of some bacteria.

The Second Line of Defence

The second line of defence is fever, phagocytosis and inflammation.
The mechanism of the second line of defence is also not specific.

Fever

Fever is the second line of defence mechanism that fights infections.
Fever increases phagocytic activity and fights against microorganisms that infect the body.

Phagocytosis

The phagocyte is a leucocyte that can carry out phagocytosis.
Neutrophils and monocytes are phagocytes.
Phagocytosis is the process by which microorganisms or other particles such as dead cells are trapped and digested by phagocytes.
When an infection occurs, the phagocytes move to the infected area and enter the tissue fluid through the pores of the capillary wall.
When a phagocyte encounters a pathogen, the phagocyte will engulf the pathogen and the lysozyme in the phagocyte will digest the pathogen.

Inflammation

Inflammation is the immediate response that destroys and neutralises harmful actions of microorganisms and toxins at the early stages of infection.
The inflamed area will swell, turn red and feel painful.
The response to inflammation is as follows:
- The damaged tissue releases histamine.
- Histamine will stimulate an immediate inflammatory response.
- Histamine will cause the blood capillaries to expand for more blood to flow to the infected area.
- Histamine also increases the permeability of blood capillaries to phagocytes.
- The phagocytes and clotting factors will accumulate in the infected area.
- The phagocytes carry out phagocytosis.

The Third Line of Defence

If pathogens overcome the second line defence, the third line of defence will be activated.
The third line of defence is the immune response of the lymphocytes.
The lymphocytes formed in the lymph nodes produce antibodies.
The actions of antibodies are specific.
Each type of antibody can only combine with a certain type of antigen.
When a person is infected with pathogens, a large number of lymphocytes will accumulate in the lymph nodes to destroy the antigens and foreign particles.
This will cause the lymph nodes to swell.
The lymph nodes also contain macrophages that destroy bacteria, dead tissues and foreign particles through phagocytosis.
Lymphocytes are divided into two types, T lymphocytes and B lymphocytes.
The T lymphocyte attacks cells infected by pathogens.
T lymphocytes stimulate B lymphocytes to produce memory cells.
If the same pathogen attacks, the memory cells will be stimulated to produce antibodies immediately.