LIWA

Aids to Health

ICSE Class 9 · Biology · Chapter 18

What this chapter covers

Key definitions

A pathogen is a micro-organism or parasite that causes disease in its host.

Aids to health are the measures and substances that help the body prevent or overcome disease when its natural defences are not enough.

An antigen is any foreign substance, such as a protein on the surface of a bacterium or virus, or a toxin, that the body recognises as not its own and that triggers the production of antibodies.

An antibody is a Y-shaped protein made by certain white blood cells (lymphocytes) in response to a specific antigen, which binds to that antigen and helps to destroy or neutralise it.

A toxin is a poisonous substance produced by a living organism, such as a bacterium or a snake, that damages the tissues of the body.

Immunity is the ability of the body to resist a particular disease by destroying the pathogen or neutralising its toxin before the disease can develop.

The local defence system is the set of physical, chemical and cellular barriers at the body's surfaces and openings, together with the inflammatory reaction at the site of an injury, that prevents pathogens from entering the body or stops them from spreading after entry.

The skin is the body's largest physical barrier.

Mucous membranes are the moist linings of the nose, mouth, windpipe, gut and other internal passages that open to the outside, and they secrete sticky mucus that traps dust and micro-organisms.

Lysozyme is an enzyme present in tears, saliva and nasal secretions that kills many bacteria by breaking down their cell walls.

Inflammation is the local response of living tissue to injury or infection, marked by redness, heat, swelling and pain, in which blood flow increases and white blood cells collect at the site to destroy the invading micro-organisms.

Phagocytes are white blood cells, such as neutrophils and macrophages, that move to the site of infection, engulf micro-organisms and dead cells, and digest them.

Key points

Questions answered

Define antigen and antibody. How are the two related?
An antigen is a foreign substance, such as a surface protein of a pathogen or a toxin, that stimulates the body to make antibodies. An antibody is a protein made by lymphocytes that binds to a specific antigen and helps destroy or neutralise it. Each antibody fits only the antigen that caused its production.
What is meant by the local defence system of the body? List three of its components.
It is the set of non-specific barriers and reactions at the body's surfaces and at wound sites that keeps pathogens out or stops them spreading. Components include the intact skin, mucus and cilia in the air passages, lysozyme in tears and saliva, hydrochloric acid in the stomach, and inflammation with phagocytes.
State three merits of the local defence system.
It is present from birth and needs no medicine; it acts immediately at the point of entry; it is non-specific, so it acts against all kinds of pathogens; and it confines an infection to a small area without side effects.
Why does an infected wound become red, warm and swollen? What is pus?
Chemicals such as histamine widen the blood vessels, bringing more blood (redness and warmth), and make capillaries leaky, so fluid and white blood cells enter the tissue (swelling). Pus is a collection of dead phagocytes, dead bacteria, tissue debris and fluid.
Distinguish between vaccination and immunisation.
Vaccination is the act of introducing a vaccine into the body. Immunisation is the process of the person becoming immune, the result that vaccination usually produces; it can also be achieved by injecting ready-made antibodies.
Explain how a vaccine protects a child against a disease years after it was given.
The vaccine's antigens cause the child's lymphocytes to make antibodies and long-lived memory cells. When the real pathogen enters later, the memory cells recognise it at once and produce a rapid, strong secondary response that destroys the pathogen before the disease develops.
What is a toxoid? Name one disease prevented by a toxoid vaccine.
A toxoid is a bacterial toxin made harmless by treatment with formalin or heat, which can still act as an antigen and cause the body to produce antitoxin. Tetanus (or diphtheria) is prevented by a toxoid vaccine.
Why is antivenom, and not a vaccine, given to a person bitten by a cobra?
A vaccine takes one to three weeks to produce antibodies, but cobra venom can kill within hours. Antivenom contains ready-made antibodies from immunised horses, which neutralise the venom immediately.