Title: The Immune System

Key words: lymphocytes, reticulo-endothelial system, bone marrow, spleen, immunity, passive immunity, acquired immunity, cellular immunity toxoids, antibodies, antigen, T-cells, B-cells, allergy, cancer

Date: July 2000

Category: 6. The Body

Type: Article

Author: DJE Candlish

The Immune System

Introduction

The immune system has evolved to protect us against infection with micro-organisms and to remove abnormal, diseased or damaged cells. Lymphocytes and other cells found in the reticuloendothelial system, the lymph glands, bone marrow, liver and spleen engulf and destroy any 'foreign' (non-body) cells, such as micro-organisms and defective or damaged body cells by a process known as phagocytosis.

The body has other protective systems, intended to prevent the entry of micro-organisms. Only when these fail does the immune system comes into play. These other systems include:

Natural immunity

Our non-specific resistance to infection by micro-organisms is known as natural immunity. Other types of natural immunity are due to genetic differences between individuals, races and species. A micro-organism that causes infection in one species may have no effect on another. Poliomyelitis and leprosy, for example, only appear to affect human beings, while distemper seems to be restricted to animals.

Acquired immunity

The concept of immunity or resistance to infection was first discovered by the ancient Chinese, who observed that people who had smallpox and recovered from it rarely caught it again. This is called acquired immunity. Chinese physicians deliberately infected children with milder strains of smallpox to make them immune to the otherwise potentially fatal strains of the disease.

This knowledge gradually spread to the Western world from the early 1700s. In 1798 a physician named Edward Jenner discovered that cowpox could be used in the same way to protect against smallpox. As cowpox was a much milder disease in humans, Jenner's technique, called vaccination ('vacca' is the Latin for cow), rapidly became the standard method of immunisation.

Pasteur took the process a stage further in the next century by weakening other micro-organisms and using these 'attenuated' strains to stimulate immunity to other diseases. More sophisticated forms of this method are still used for modern vaccination programmes, using killed or attenuated bacteria or viruses or using substances (toxoids) produced by killed micro-organisms.

Active acquired immunity is the result of an antigen stimulating the body to form a specific antibody, which then neutralises the antigen.

An antigen is usually some form of foreign protein, such as the cell walls of bacteria or viruses. Lipids, complex carbohydrates and other organic substances can also act as antigens.

An antibody is a specific type of protein (g - or immuno-globulin). This protein bonds to the antigen in the antigen-antibody reaction, forming a harmless complex. Each antibody is specific to only one antigen.

The dead disease organisms used for vaccination are killed by chemicals which do not destroy their antigenic properties, so they cause the formation of antibodies without causing the disease. Killed vaccines are easily produced and are used for diseases with short-term immunity. Examples of the organisms involved include:

 

In a similar way, toxoids produced by certain disease-causing organisms can be treated to retain their antigenic properties while losing their toxic effects. Examples include tetanus toxoid, used to prevent lockjaw.

Active immunity takes about 4-6 weeks to develop. Sometimes a second vaccination, or booster, is needed to produce long-term immunity.

Passive immunity

Passive immunity involves the administration of preformed antibodies from one person or animal into another. In this situation, the individual receiving the antibodies does not produce any antibody of their own. An antiserum is a preparation containing large quantities of antibodies to a specific disease and is usually prepared from the blood of immunised cows or horses. Antisera are used when vaccines are unsuitable, such as when immediate protection is needed. Tetanus antitoxin may be given to someone after a road accident, for example, to protect against tetanus infection. As well as infectious diseases, antisera are available for certain snake and insect bites.

Passive immunisation also happens during development of the foetus, with antibodies passing from the mother to the foetus through the placenta and, after birth, from the mother's milk to the baby. This is one reason why breast feeding is better than bottle-feeding. The effects of passive immunisation are short-lived, as the antibodies are gradually broken down in the body.

 

 

The different forms of immunity

IMMUNITY

natural

active

acquired

Passive

infection and recovery

vaccination

from the mother in the womb

Administration of preformed antibodies

Humoral and cellular immunity

The scientific study of immunology has revealed that immunity depends on two types of cell:

The B-cells produce the antibodies that circulate in blood plasma and make the antigen-antibody reaction possible. This is known as humoral immunity. There are five classes of antibody, based on the structure of the g -globulin or immunoglobulin (Ig) involved. These are IgA, IgD, IgE, IgG and IgM. IgE is involved in allergy, while IgG and IgM are the main antibodies against bacteria and viruses.

The T-lymphocytes play a different role in immunity. They cause cellular immunity. Two different types of T-cell are involved, effector T-cells and regulator T-cells. Effector cells react immediately against antigens, by attacking virally infected body cells, for example. The regulator T-cells play a more indirect role, by influencing B-cells.

T-helper cells activate the B-cells, while T-suppressor cells stop the B-cells producing antibodies. Both B-cells and T-cells also form memory cells in response to antigen. These 'remember' antigens and allow for more rapid production of T- and B-cells if the body contacts the same antigen again in future.

As the study of immunology has progressed, it has become clear that the immune system is also involved in a number of other clinically important biological processes, such as allergy and cancer. This is also shown by HIV infection, which attacks the immune system and makes the human body vulnerable to many infections that would normally cause no problems and can lead to the development of unusual cancers.

Allergy and the immune system

Allergy can be regarded as a faulty immune reaction. Although the immune system is intended to destroy micro-organisms, it sometimes reacts to foreign proteins like pollen, animal fur (danders), mould spores, housedust mite proteins and even foods, such as peanuts, gluten or milk. The antibodies produced, usually IgE or IgG, then cause the symptoms of allergy. In non-atopic (non-allergic) individuals, exposure to these substances has no effect.

These symptoms vary according to where the allergic or antigen-antibody reaction occurs. In the nose, for example, the familiar symptoms of hayfever are produced - sneezing, itching and running. The eyes are also often involved, with redness, itching and watering.