Our immune system

The immune system is responsible for the unenviable task of protecting the body against invisible foes.

We have to coexist with countless number of microorganisms of various types-viruses, bacteria, fungi etc.

They can invade the body through the skin, air ways or the digestive system.

The system protects the body in various ways and at different levels. It is endowed with the intelligence to distinguish between self and non self. Hence it can destroy microorganisms trying to cause disease but does not harm normal body cells.

Immunity is the resistance to diseases and can be innate or acquired, active or passive, local or systemic, specific or nonspecific.

First level security: This is provided by the layers of skin and the epithelium (inner lining) of respiratory system, digestive system and urinary tract and is non-specific in nature. Our skin has innumerable number of bacteria on it.

Normally they cannot invade the superficial layer of skin which is made up of dead cells. If the integrity of skin is broken then the bacteria enter the deeper layers causing disease. Sebum, an oily substance secreted by the sebaceous glands in the skin also has antibacterial actions.

The air we breathe contains dust and many microorganisms. The hairs in the nose filter dust particles. The inner lining of the airways is covered by a viscid fluid called mucus which traps dust and microorganisms and expels them.

The saliva contains antibacterial substances which help to keep the mouth clean. The stomach secretes a juice which is highly acidic in nature. This juice can destroy microorganisms and prevent them from going further down.

Tears produced by the eyes also has antibacterial action.

The vaginal secretion is also acidic and prevents growth of disease causing microorganisms.

A pregnant woman transmits many antibodies to the fetus in the uterus through the placenta. Hence a newborn baby is protected against the common infections in that area till his immune system develops.

Breast milk contains many antibodies and cells which protect the baby. Hence breast feeding can protect the baby against many infections. Both are examples of passive immunity.

As a child grows older he gets exposed to many infections through air, water and food.

It any microorganism breaks the first tier security the cells in the blood immediately launch an attack on it. The white blood cells in the blood are responsible for protecting the body against infections. They do it in different ways.

Phagocytosis: When the white blood cells recognize a microorganism they try to swallow it up and then destroy it with the help of enzymes. Neutrophils and monocytes are capable of phagocytosis. This is the second level security.

Lymphocytes are not capable of phogocytosis. But they launch two pronged attacks on micro organism in the form of humoral immunity and cell mediated immunity. They are like special commandoes and provide the third level of security.

There are two types of Lymphocytes- T cells and B cells. B cells are responsible for humoral immunity while T cells are responsible for cell mediated immunity.

Humoral immunity: This consists of production of antibodies specific against a particular antigen. An antibody attaches to and forms a complex with the antigen in the same way interlocking blocks are joined.

Antibodies recognize and trap antigens floating in the bloodstream. But they cannot penetrate cells. There are microorganisms which can thrive inside the cells.

Each B cell makes one type of antibody active against a particular antigen. When exposed to an antigen the B cell undergoes a transformation and becomes a plasma cell which goes on dividing forming a cluster of plasma cells all producing the same type of antibodies.

Cell mediated immunity: This type of immune response does not involve production of antibodies but consists of activation of macrophages, natural killer cells and release of certain chemicals called cytokines in response to an antigen. Cytokines act as intercellular mediators and help in coordinating immune responses.

Cell mediated immune response is essential for killing intracellular microorganisms and cancerous cells.

T cells responsible for cell mediated immunity cannot recognize free floating antigens in bloodstream but have special receptors which detect antigens on the surface of infected or cancerous cells.

T cells act in two ways. Helper T cells communicate with other cells. Some helper cells stimulate B cells to produce antibodies. Some helper cells activate phagocytes and other T cells.

Killer T cells (Cytotoxic T Lymphocytes or CTLs) attack cells having antigens recognized as non-self by the immune system. Every cell in our body contains protein molecules called Major Histocompatibility Complex (MHC) which is unique to every individual.

All cells possessing a particular MHC are recognized by the immune system as self and accepted. If the MHC is modified in any way either due to infection or cancerous change such a cell is attacked and destroyed by the Killer T cells. They possess certain granules containing chemicals for this purpose.

Natural Killer cells (NK) are special lymphocytes containing similar granules. These cells look for cells which do not contain the self MHC molecule and attack them.

Both Killer cells attach to cells recognized as non-self, release their lethal chemicals destroying such abnormal cells.

Lymphatic system: The lymphatic system consists of a network of tube like structures called lymphatic vessels (somewhat like blood vessels but having different structure) and lymphoid tissues. It is an important component of the immune system.

As blood passes from arteries to veins through capillaries a small amount of plasma leaks out of the capillaries and enters the space between the cells (interstitial space).

The fluid in the interstitial space is called the interstitial fluid. It is through the interstitial fluid that cells get nutrients and excrete waste products.

Most of this fluid which oozes out of the capillaries gets back into the blood stream at the venous end. The rest of this fluid enters the lymphatic capillaries which open into the interstitial space. This fluid is called lymph.

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The lymphatic system is not a closed one. There are valves inside the lymphatic vessels to prevent the fluid from leaking back into the interstitial space.

Lymph drains into progressively larger vessels which culminate in two large tubes called the right lymphatic duct (for lymph from the right upper body) and the thoracic duct for lymph from the rest of the body.

Both these ducts ultimately join the circulatory system by emptying lymph into large veins in the neck called subclavian veins.

Lymphoid tissue is responsible for the immune response required to protect the body against infections and spread of cancerous cells. It consists of fibrous tissue with white blood cells enmeshed in it, the most abundant being lymphocytes.

Primary lymphoid tissues are concerned with production of lymphocytes and include the thymus and bone marrow. Secondary lymphoid tissues provide a platform for exposing the immune cells to antigens. They can be encapsulated and localized or un-encapsulated and diffuse.

Lymph nodes are encapsulated lymphoid tissues and are found along the path of lymphatic vessels.

Lymph enters the lymph nodes through afferent lymph vessels, percolates the lymph node and leaves the nodes through efferent lymph vessels. The efferent vessels leave the nodes from a depressed area on the node called the hilum.

In the small intestine the lymphatic vessels carry fatty acids which are formed after digestion of dietary fat along with lymph. This fluid is called chyle and has a milky appearance.

If any microorganism enters the body by breaking the first level of security it is carried by lymphatic vessels to the lymph nodes where the competent cells mount an immune response. That is why lymph nodes are enlarged whenever there is infection in the draining area.

The spleen is also a lymphoid organ. It is situated in the left upper abdomen between the stomach and the diaphragm. It also acts as a reservoir of blood. It removes dead cells from the blood and also supplies white blood cells. If the spleen is removed for any reason the person becomes susceptible to infections.

The complement system: This is a group of proteins which help antibodies to destroy microorganisms. It also helps the body to remove antigen-antibody complexes. The complement proteins play a role in initiating inflammation which causes redness, swelling, pain and warmth.

The complement proteins circulate in the blood in inactive form. When the first protein in the series is activated usually by an antigen-antibody complex it in turn activates the next protein starting a chain reaction known as the complement cascade resulting in the formation of proteins which carry out the above functions.

Keep your immune system fighting fit

The immune system is like a very sophisticated and versatile military force which keeps the invisible foes at bay. It can launch highly specific and targeted attacks against the micro-organisms which try to cause disease and the cells which have undergone cancerous change.

It usually works with greater accuracy than the laser guided bombs. Without the proper functioning of the immune system survival is impossible. If the system goes berserk it can cause damage to the body resulting in so called auto-immune disorders.

Hence you should to maintain the immune system in good health by consuming healthy diet. Deficiencies of some vitamins and minerals can impair the functioning of the immune system.

Cigarette smoking and alcohol are known to adversely affect the immune system.

Mind has tremendous influence over the body. Mental status can affect all the organ systems of the body. Hence it is very important to have healthy mind and keep it free from negative feelings. Regular meditation can help to maintain sound mental health.

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