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Our body is considered as a suitable host for the growth of a number of micro-organisms and can affect our life if allowed to multiply within the body. The complex network of cells, tissues and organs are collectively referred to as the immune system.

Basically speaking, the immune system functions as a defense system, which deters harmful invaders such as bacteria, viruses, yeasts, fungi and protozoa from invading the body. The immune system can clear harmful micro-organisms as well as resulting toxins, and is necessary to prevent the occurrence of a number of infections in day-to-day life. While the immune system functions in an excellent manner in many of the cases, it may begin to misfire at times leading to disorders in our body.

The Harmony of Immune System

The immune system is formed by several different organs, cells and tissues which work in harmony to keep the body healthy and free from infections. The immune system is mainly composed of certain specialized organs that are referred to as lymphoid organs. These organs are responsible for the production and maturation of the specific cells known as the white blood cells that circulate in the blood and fend off infectious microorganisms. Some of the organs that produce immune cells include:  tonsils and adenoids, thymus, spleen, and bone marrow of long bones (such as the hands and legs). These are assisted by a number accessory structures referred to as lymph nodes. The immune cells produced by these organs are collectively known as leukocytes or the white blood cells.

These white blood cells are basically divided into two major types:
  1. Granulocytes (which is comprised of cells known as neutrophils, eosinophils and basophils) which can be collectively referred to as phagocytes
  2. Agranulocytes: which include cells known as lymphocytes (such as B cells/lymphocytes, T cells/lymphocytes and natural killer (NK) cells), monocytes and macrophages
These cells are assisted by a number of proteins and chemical messengers known as receptors, cytokines and the complement system. A highly complex communication occurs amongst the cells of the immune system which enables them to attack an invading organism and kill it.1, 2 

Mode of Action of the Immune System and Destruction Process

The immune system works at different levels, much the same as defense systems that defend our countries. The cells of the immune system may become activated based on the type of organism invading our body. Phagocytes are the first line of defense against common infections caused by bacteria. Once a harmful bacterium is identified the specific tissues such as the bone marrow begin to produce more and more phagocytes in order to kill the bacteria.

Among the agranulocyte cells, the B-cells and T-cells play a vital role in identifying and destroying more lethal organisms such as viruses. The B-cells have been compared to the intelligence wing in the defense system which identifies harmful microorganisms and marks them to be destroyed by the T-cells ( simply referred to as the soldiers).1, 2, 3

The particular pattern in which the B-cells identify and mark the harmful organisms can be explained by the antigen-antibody formation. Simply put, both antigens and antibodies are certain specialized proteins. The antigens are proteins present on the outer surface of the harmful microorganisms. Antigens may also be present on blood cells (which is responsible for the rejection of blood in case of blood group mismatch), organs (if not matched can cause rejection of organs instances of organ transplants) and toxins. The T-cells and other cells responsible for killing the harmful microorganisms may not be able to identify the antigens until and unless they are marked by the B-cells. The presence of an antigen elicits the production of another specialized protein by the B-cells which is known as antibody. The antibody produced by the B-cells for a particular antigen is highly specific and gets attached to the antigen. This antigen-antibody complex helps the T-cells to easily identify and destroy the microorganism. The cytokines, the complement system, and the phagocytes all function together in the destruction process.1, 2

Immune Memory

There is another interesting aspect about the antibodies, i.e., memory. Once formed, the body begins to produce similar antibodies throughout life which helps to defend against similar attacks. However, as previously mentioned these antibodies are highly specific to a particular type of antigen. The antibody cannot recognize the same organism even if it has undergone minor changes in its antigen structure.4

Types of Immunity

This protection offered by the immune system is commonly referred to as immunity. Further, immunity can be categorized into three types: innate, adaptive and passive. Passive immunity refers to the type of immunity that tends to be present for a short duration of time and is generally borrowed. The mother’s breast milk provides passive immunity to the newborn child until the immune system is developed. Innate immunity refers to the immunity present since birth, and can also be referred to as natural immunity. Adaptive immunity as the name suggests is an adaptation that occurs in the immune system when the body is exposed to different kinds of microorganisms as life progresses. Vaccination can be cited as the most common way of inducing adaptive immunity. Vaccination involves the introduction of harmful microorganisms in very mild forms that are capable enough to induce the formation of antibodies. Such antibodies once formed, can fight off the actual infection in a highly capable manner.

Influencing Factors

The immune system of each individual is unique and has varying ability to fight the infecting organism. The strength of the immune system may vary based on a variety of factors such as the type of environment an individual is born and brought up, his genetic patterns, exposure to different microorganisms, lifestyle, diet and numerous other factors. The ability of the microorganism to infect us depends on the ability to either completely avoid being recognized by our immune system or its ability to destroy certain key components of the immune system (such as the HIV which destroys the T-cells).2, 4

Immune System Related Conditions

The immune system can be affected or may function erratically at times. Several disorders are associated with the alteration in the functioning of the immune system and are grouped under the following headings 2:

Immunodeficiency Disorders: where the immune system is not capable of overcoming the infectious organism due to deficiency in number or quality

Autoimmune Disorders: wherein the body’s immune system begins to attack its own cells
Allergic Hypersensitivity Reactions: the immune system reacts in a highly magnified way so as to destroy the normal cells during the immune reactions

Cancers: the organs responsible for production of immune cells may be affected by cancers that results in overproduction of incompetent immune cells