Immune System Notes

Why are children, especially toddlers and infants, more prone to infections than teens and adults?

Yes, this is due to the immune system.

Children, both infants and toddlers tend to fall ill more often and are more prone to infections as they do not have a fully developed immune system, compared to adults. Likewise, as people grow old, their immune system weakens and makes things worse.

Everyone’s immune system is different and it varies with age and lifestyle. There are many factors which affect the immune system.

Here, in this article let us learn in detail about the human immune system, its different parts, types and other facts related to it.
Table of Contents

What is the Immune System?

The Immune System consists of different types of cells and molecules, which protect our body against pathogens. Pathogens are defined as everything from parasites to fungi, bacteria, viruses and haptens. Haptens are molecules that may cause an immune response when they come in contact with a protein. All these cells and molecules are distributed in all the tissues of the body as well as lymphoid organs, which eliminate or prevent microbial infections to decrease the growth of tumours and start the repairing process of damaged tissues.

Read More: Immunity

Types of Immunity

Immune System fights against microbes and is divided into different types of reactions. The three types of immunity are:

Innate immunity

We all are born with innate (or natural) immunity, a type of natural protection.

The immunity, which is present in an organism by birth is called innate Immunity. Innate immunity refers to the body’s defence system.

For example, the skin, the barrier of the human body functions by protecting the entry of germs and other disease-causing pathogens.

Adaptive immunity

An individual acquires immunity after birth, hence is called adaptive or acquired immunity. Adaptive immunity develops throughout our lives. We acquire adaptive immunity when we are exposed to infectious diseases and allergies or when we are immunized against them with vaccines.

Passive immunity

Passive immunity is developed by antibodies that are produced outside the body. This immunity lasts for a short time. For example, antibodies present in a mother’s breast milk provide a baby with temporary immunity against diseases.

Active Immunity

Active immunity is a type of adaptive immunity, which is developed due to the production of antibodies in one’s own body. This type of immunity occurs when we are in contact with the pathogen or its antigen. When the immune system encounters an antigen for the first time and active immunity responds against it, it is called a primary response. Once a body experiences a pathogen for the first time, it keeps a few of the antibodies that attacked the pathogen just in case it attacks for the second time. This is known as natural active immunity.

Also Refer: Difference Between Active and Passive Immunity

What are the parts of the Immune System?

The main parts of the immune system are:

  • Spleen.
  • Thymus.
  • Antibodies.
  • Bone marrow.
  • White blood cells.
  • Lymphatic system.
  • Complement system.

Disorders of the Immune System

Some of the common disorders caused by a poor immune system include:

  1. Allergic diseases – These include hay fever, sinus disease, asthma, hives, dermatitis and eczema.
  2. Autoimmune diseases – These include multiple sclerosis, autoimmune thyroid disease, type 1 diabetes, systemic lupus erythematosus, rheumatoid arthritis and systemic vasculitis.
  3. There are immunodeficiencies, which are inherited from a parent to the child. These conditions include primary immunodeficiency diseases such as x-linked severe combined immunodeficiency (SCID), complement deficiencies, common variable immunodeficiency (CVID), etc

Also Refer: Allergies and Autoimmunity

This article concludes an introduction to the human immune system.

To know more about the immune system, its parts, types, functions, its disorders, other related topics and important questions, keep visiting our website at BYJU’S Biology.

Frequently Asked Questions


Differentiate between: Active Immunity and passive immunity.

Active Immunity: Immunity developed by an individual as a result of a previous infection or in response to an antigen which enters the body naturally or is introduced artificially

Passive Immunity: Immunity provided to an individual from an outside source in the form of readymade antibodies.


Differentiate between: Innate immunity and acquired immunity

Innate immunity: It is the natural immunity by virtue of genetic constitutional makeup.
Acquired immunity: It is the resistance to a disease, an individual acquires on exposure to pathogens or vaccines.

What is immunization?

Immunization can be defined as the process by which an individual is made immune to an infectious disease by the administration of an external agent or vaccine. On administration of the vaccine, the body starts developing antibodies against it and gets immunized from the disease. On further exposure to the similar infectious agent, the body fights the infection with the help of pre-existing antibodies.

Why is the immune system essential for our health?

The immune system of our body is a kind of defence mechanism to fight against pathogenic microbes. It has cells that are specialised to kill infecting microbes and keep our bodies healthy.
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