Immune System

What is the Immune System?
The immune system is also called as the defense system, of the body. Our body is subjected to bacteria, viruses, parasites and fungi throughout our lifetime. These organisms are usually found in the skin, respiratory tract, mouth, intestinal tract, eyes and urinary tract. Many of these agents can cause serious damage if they invade the deeper tissues. In addition to these, there are other organisms present that are capable of causing lethal diseases, like pneumonia and typhoid. But we are not always ill, are we? This is thanks to a special mechanism, which combats these organisms and toxic agents. This mechanism is called as the immune system of the body. It consists of blood and the lymphatic system of the body.

Where is the immune system located and what does it consist of?
The immune system is located throughout our body in various forms. There are two main mechanisms of the immune system.

Physical and chemical mechanism
This consists of the physical barriers in the body. These are the:
• Skin, which contains sweat and sebaceous glands that trap these organisms on the surface and contain a mild antiseptic agent.
• Mucus in the respiratory tract traps the mucus and the fine hair or cilia move the debris to the throat from where it is swallowed or coughed out.
• Saliva contains a mixture of mucus and chemicals called enzymes, which cleanse the mouth.
• Tears wash away dirt and contain an antiseptic substance.
• Hydrochloric acid in the stomach secreted by gastric cells kills most invading organisms on contact.
• Goblet cells in the intestinal lining also produce protective mucus.
• There is presence of harmless bacteria growing on the surface of the body as well as the digestive tract, which prevents the growth of other harmful bacteria.

Cellular mechanism
One of the components of blood are cells called leukocytes. These cells are also called as the white blood cells and are the soldiers of our body. These cells work in two ways:
1. They cause destruction of the invading agents by engulfing them, which is called phagocytosis.
2. They can also form structures called as antibodies and sensitized cells that can destroy the invader.
Leukocytes are of five types :
• Neutrophils
• Eosinophils
• Lymphocytes
• Basophils
• Monocytes

In addition to leukocytes, there are platelets present in blood. These platelets are attracted at the site of tissue injury. They plug the site of bleeding and stimulate certain clotting factors in the blood which clots blood and prevents bleeding.

How do these cells protect the human body?

The Inflammatory Response
Sometimes foreign organisms such as bacteria invade the body overcoming its physical and chemical barriers. Then the cellular mechanism comes into play. As soon as the tissue is damaged, it releases specific chemicals. These chemicals then attract the white blood cells called ‘phagocytes’. The region at which these chemicals are released has underlying blood vessels or capillaries that widen and the blood flow increases. This causes a characteristic reaction seen from outside. Redness, pain, heat and swelling is produced at the damaged site. This is called as the inflammatory response.

The blood vessels after swelling become porous and allow the leukocytes to enter them and reach the foreign particle. Then the white blood cell engulfs the particle slowly spreading its arms around it. It also produces enzymes, which then destroy the particle.

The Antibody Immune Response
This specific immune response targets invading bacteria and relies on white blood cells called ‘B-lymphocytes’. The invading organisms have a particle on their surface that is a protein by nature. This is called as an ‘antigen’. The B-lymphocytes recognize this protein the first time they encounter it. Then they multiply to form two types of cells: the ‘plasma cells’ and the ‘memory B cells’.

This process is called sensitization. The plasma cells form substances called antibodies. The memory cells are stored in the body for future use. When the B cells encounter the same protein for a second time, the B memory cells are activated. Then they rapidly multiply to form plasma cells forming more antibodies. These antibodies seek out the bacteria, lock on to them and then inactivate them. The antibodies also attract more phagocytes to the site of invasion to destroy the bacteria.

The Cellular Immune Response
This type of specific immune response targets viruses, cancer cells and parasites. It is dependent on white blood cells called as T cells. The T cells recognize the antigen on the virus. They then multiply to form two types of cells called ‘killer T cells’ and ‘memory T cells’. Killer T cells produce toxins, which inactivate the virus.

On second exposure to the same antigen causes the memory T cells to multiply and form killer T cells. These then seek out and lock on to the antigens and release their toxic proteins. These proteins then destroy the infected cell.

Allergic Reactions
An allergy is a response to a normally harmless substance called as an allergen. This allergen may be a substance like pollen, fur, dust, or a particular kind of food. On initial exposure the antigen on the allergen sensitizes the immune system thus forming antibodies. Subsequent exposure to the same antigen causes these antibodies to bind to the surface of mast cells, which are present in the skin. These allergens bind and link two or more antibodies, causing the cell to burst. The burst cell releases a substance called histamine that causes an inflammatory reaction, producing allergic symptoms.