Biology and Health Sciences

Key unit competency 

To be able to describe natural and artificial methods that fight against infection.

Learning objectives 

After studying this unit, I should be able to: 

• Explain how each pathogen has its own antigens. 

• Define active immunity, pathogen and antibody production in the body. 

• Demonstrate  the importance of passive immunity for breastfed infants. 

• Use a diagram to illustrate antibody-antigen reaction sites. 

• Observe and differentiate the immune responses. 

• Advocate for vaccination and breastfeeding as a sustainable disease prevention method.

Introduction 

When pathogens find their way into the body, they can cause diseases. However, the body has its own mechanisms of dealing with such foreign materials. The Ability of the body to defend itself against foreign bodies and disease-causing microorganisms is known as immunity. The system that is responsible for defending the body against diseases is known as the immune system. Look at the picture below. What is happening in the picture? 

13.1 Antibodies and antigens 

Activity 13.1: Investigating antibodies and antigens

i)  A chart showing an antigen and antibody will be displayed on the wall. 

ii)  Observe the diagrams on the chart carefully and identify the following: 

Study questions 

a)  Why are antibodies specific to only one type of antigen? 

b)  Draw an antibody. 

c) Describe the antibody-antigen complex.

An  antibody  also known as an immunoglobulin is a large Y-shaped protein. It is produced mainly by plasma cells. It is used by the immune system to identify and neutralise pathogens such as bacteria and viruses. The antibody recognises a unique molecule of the harmful agent, called an antigen. The antibody then binds to this antigen. Antibodies are specific to their antigens.

Antigen 

This is any substance that is capable of stimulating an immune response, specifically activating the body to produce antibodies. In general, two main divisions of antigens are recognised: foreign antigens and self-antigens. 

• Foreign antigens come from outside the body. All pathogens are examples of foreign antigens.  Viruses and micro-organisms such as bacteria and protozoa, and certain proteins in foods all lie under this group. 

• Self-antigens, on the other hand, come from within the body. Normally, the body is able to distinguish self from non-self. However, in individuals with autoimmune disorders, normal body substances provoke an immune response, leading to the generation of auto-antibodies. 

Antigen–antibody reaction 

An antigen and antibody reaction works like a lock and key mechanism. Antibodies themselves do not destroy antigens. They inactivate and tag antigens for destruction by phagocytes. 

All antibodies form an antigen-antibody (immune) complex When they combine with antigens. 

Each antigen stimulates the production of a specific antibody. All antibodies are Y-shaped. They however, differ at the antigen binding site. Each antibody has a site complementary to a certain antigen. This means that each antibody can only bind to a specific antigen.

In response to the antigen, the antibody wraps its two sites like a ‘lock’ around the ‘key’ of the antigen sites to destroy it. An antibody's mode of action varies with different types of antigens. With its two-armed Y-shaped structure, the antibody can attack two antigens at the same time with each arm. 

Defensive mechanisms used by antibodies are neutralisation, agglutination, precipitation and plasma complement system. 

i) Neutralisation – antibodies bind to and block specific sites on antigens; viruses and bacteria. This prevents antigens from binding to sites called receptors on tissue cells.  They are later destroyed by phagocytes, for instance, if the antigen is a toxin produced by pathogenic bacteria that cause an infection like diphtheria or tetanus, the binding process of the antibody will neutalise the antigen's toxin. 

ii)  Agglutination – antibodies bind the same determinant on more than one antigen. An example is when an antibody surrounds a virus, such as one that causes influenza, it prevents it from entering other body cells. 

iii) Precipitation – soluble molecules are cross-linked into large insoluble complex. After which they fall out of solution, and are phagocytised. 

iv) Plasma complement system- The antibodies coat infectious bacteria and then white blood cells will complete the job by engulfing the bacteria, destroying them, and then removing them from the body. 

Self-evaluation Test 13.1 

1. Draw a key and lock and relate it to antibody- antigen reaction. 

2.   How does the body defend itself from infections?

13.2 Immune response

Activity 13.2: Observing immune response 

You will be provided with computer aided simulations of the immune response of organisms. 

1. Observe the simulations carefully. 

2.  Differentiate immune responses of organisms. 

3.  Share your findings with the class.

Immune response refers to the process by which the body recognises and defends itself against bacteria, viruses and substances that appear foreign and harmful. The immune system is a collection of cells, tissues and molecules that protect the body from numerous pathogens and toxins in our environment. 

This defense against microbes has been divided into two general types of reactions: 

 a) Innate immunity 

Innate immunity also called nonspecific immunity is the body defense system that one is born with. It protects against all antigens. Innate immunity involves barriers that keep harmful materials from entering your body. It consists of cells and proteins that are always present and ready to fight microbes at the site of infection.

The main components of the innate immune system are: 

i. Physical epithelial barriers 

ii. Phagocytic leukocytes 

iii. Dendritic cells 

iv. A special type of white blood cells called a natural killer (NK) cell 

v. Circulating plasma proteins

Innate immunity includes the components of the body’s first line of defence. These are enzymes in tears and skin oils, mucus, skin and acid in the stomach.If an antigen gets past these barriers, it is attacked and destroyed by other parts of the immune system. 

b)  Adaptive immunity 

Adaptive immune system also known as acquired immunity is immunity that develops after exposure to various antigens. The immune system builds a defense against that specific antigen. It is called into action against pathogens that are able to evade or overcome innate immune defenses.  Components of the adaptive immune system are normally silent; however, when activated, these components “adapt” to the presence of infectious agents by creating mechanisms for neutralising or eliminating the microbes. 

The immune system includes certain types of white blood cells, for example, lymphocytes.  There are B and T type lymphocytes. 

• B lymphocytes are cells that produce antibodies. Antibodies attach to a specific antigen and present it to other immune cells for destruction. 

• T lymphocytes are cells that attack antigens directly and help control the immune response. 

During growth and development of white blood cells, they always learn to differentiate between individuals own body tissues and foreign substances. Once B cells and T cells are formed, a few of those cells will multiply and provide "memory" for the immune system. This allows the immune system to respond faster and more efficiently the next time an individual is exposed to the same antigen. In many cases it will prevent you from getting sick, for example, a person who has had measles or has been immunised against measles is immune from getting it again. 

Active immunity 

This is the immunity that results from the production of antibodies by the immune system in response to the presence of an antigen. Active immunity is either natural or artificial. 

• Naturally acquired active immunity occurs when a person is exposed to a live pathogen. The individual develops the disease, and becomes immune as a result of the primary immune response. 

• Artificially acquired active immunity can be induced by a vaccine, a substance that contains alternated form of  the antigen. A vaccine stimulates a primary response against the antigen without causing symptoms of the disease. 

Passive immunity 

This is a short-term immunity that results from the introduction of antibodies from another person or animal. Passive immunity is either artificial or natural. 

• Artificially acquired passive immunity is a short-term immunisation.  It is achieved by the injection of antibodies that are not produced by the recipient's cells. 

• Naturally acquired passive immunity occurs during pregnancy, in which certain antibodies are passed from the maternal body into the foetal bloodstream. 

Passive immunity for breast–fed infants 

Discussion corner 

1.  Discuss with a classmate the following: 

2. Share your findings with the class. 

Infants have passive immunity because they are born with antibodies that are transferred through the placenta from their mothers. They also get some antibodies from the mother’s breast milk. These antibodies disappear between ages 6 and 12 months. Passive immunisation may also be due to injection of antiserum, which contains antibodies that are formed by another person or animal. It provides immediate protection against an antigen, but does not provide longlasting protection.  BCG given to a child at birth to protect it from TB is an example of passive immunisation.

13.3 Immunisation and autoimmunity 

Immunisation is the process by which an immune response is triggered by the administration of a vaccine towards an infectious disease. Small amounts of an antigen, such as dead viruses, are given to activate immune system "memory". Memory allows your body to react quickly and efficiently to future exposure. Such small doses of antigens are referred to as vaccines. 

Immunisation is an important method of disease prevention. By receiving a vaccination, usually in the form of an injection, a person can be 'immunised' against a disease, and reduce their likelihood of developing the illness. Immunisation can also reduce the spread of the disease in the population. 

Self-evaluation Test 13.2 

1. Differentiate between active and passive immunity. 

2.  What is immune system made up of? 

3.  Outline the importance of breastfeeding. 

Discussion corner 

1.  Discuss with a classmate how a person’s own immune system can bring about Type 1 diabetes. 

2.  Share your findings with the class.

Autoimmunity is a condition whereby the immune system attacks and kills own cell. Any disease that results from such an immune response is termed an autoimmune disease. 

An efficient immune response protects against many diseases and disorders. Wrong immune response causes immune system disorders. An overactive immune response can lead to the development of autoimmune diseases, in which antibodies form against the body's own tissues.

Complications from altered immune responses include: 

Type 1 diabetes and the immune system 

Type 1 diabetes was initially known as juvenile diabetes or insulin-dependent diabetes mellitus. It is a chronic condition in which the pancreas produces little or no insulin. Insulin is a hormone needed to enable sugar (glucose) to enter cells to produce energy. 

The pancreas is the organ responsible for controlling the amount of sugar (glucose) in the body. Specific cells in the pancreas known as the beta cells are responsible for manufacturing insulin. Insulin released into blood regulates the amount of sugar in the body keeping it at normal levels.

When a virus attacks the body, T- cells start producing antibodies to fight against this virus. However, the virus can have same antigens like those of the beta cells in the pancreas. In this case, the T-cells mistakenly recognise beta cells as foreign to the body. They attack these cells hence destroying them. Once the beta cells of the body are destroyed, no insulin is produced. This leads to an increase in blood sugar level and therefore increases the possibility of having diabetes.

Not every virus can trigger the T cells to turn against the beta cells. The virus must have antigens that are similar enough to the antigens in beta cells, and those viruses include: 

Note: Type 2 Diabetes mellitus occurs when the body cells becomes resistant to insulin or when less insulin is produced by the body.  

Self-evaluation Test 13.3 

1.  What can make antibodies turn against oneself? 

2.  Compare type 1 diabetes and type 2 diabetes.

Unit summary 

• An antibody is a protein produced by the body's immune system when it detects harmful substances, called antigens. Examples of antigens include micro-organisms (bacteria, fungi, parasites, and viruses) and chemicals. 

• An antigen is any substance that triggers the immune system to produce antibodies that fight and destroy the antigen. 

• Each pathogen has its own antigens that have specific binding sites that are only recognised by certain antibodies. 

• Antibodies lock on to antigens, thereby marking them for destruction by phagocytes. 

• Immune response is the body’s mechanism of defending itself against bacteria, viruses, and substances that appear foreign and harmful. 

• Naturally acquired active immunity occurs when the person is exposed to a live pathogen, develops the disease, and becomes immune as a result of the primary immune response. Artificially acquired active immunity can be induced by a vaccine, a substance that contains the antigen.

• Passive immunity is a short-term immunity that results from the introduction of antibodies from another person or animal.

• Immunisation is the process whereby a person is made immune or resistant to an infectious disease, typically by the administration of a vaccine. 

• Autoimmunity is a condition in which the immune system invades and kills own cells and tissues. Any disease that results from such a response is termed an autoimmune disease.