Immune System: Parts, Function, and Diseases

Immune system

You encounter infinite foreign particles daily. Isn’t it? However, not all of them lead to diseases. Thanks to your immune system that you are able to go to new places with different climates and try new foods. Your immune system is responsible for protecting you from various deadly antigens; which might result in life-threatening conditions. The immune system is a type of a host-defence system. It consists of different cells, tissues, and organs. In addition, it also encompasses several biological processes, which take place every second inside the human body, and thus, protect us from foreign agents like viruses, bacteria, parasites, etc.

Let’s discuss various functions of the immune system, parts of the body which function in immunity, the types of immune systems, and the diseases which might occur if the immune system is not working properly.

Immunology refers to the branch of biology which includes the study of the immune system. Immunology plays an important role when we want to understand various human diseases, disorders, immunodeficiencies, etc. Antigens are molecules which are able to evoke an immune response

Functions of the Immune System

The ultimate goal of the immune system is to protect the human body from foreign agents; which may be microbes or chemicals. When the immune system of a human is working efficiently, it is able to identify and distinguish a number of dangers, including viruses, bacteria, parasites, and other pathogens. Therefore, the human immune system works against each threat and maintains the integrity of the body.

Immune System in the BodyParts of the immune system

The human immune system consists of different types of lymphoid organs, tissues, cells, blood vessels; which directly or indirectly aid in rendering immunity against foreign agents.

The lymphatic organs

  • Lymphocytes are a type of white blood cells. The organs where lymphocytes originate, mature, and proliferate are known as the lymphoid organs.
  • Thymus and bone marrow are the primary lymphoid organs; where maturation of the lymphocytes occur.
  • In the secondary lymphoid organs, the lymphocytes interact with the antigen, and, in turn, proliferate into the effector cells. The secondary lymphoid organs include spleen, lymph nodes, tonsils, Peyer’s patches of small intestine and appendix.
  • The most important lymphoid organ is the bone marrow. It is the site of production of all types of blood cells, including the lymphocytes.

Functions of bone marrow

  • Both the bone marrow and the thymus are the critical sites for the T-lymphocytes; as these are the places where the development and maturation of T-lymphocytes occur.
  • The bean-shaped organ, i.e. spleen, recycles the old erythrocytes. It plays an important role in acting as a filter of the blood; and thereby, helps in the removal of the blood-borne microorganisms.
  • The lymph travels across the human body in vessels known as lymph vessels. The lymph nodes filter lymph and store white blood cells. The lymph nodes trap the foreign pathogens and antigens and, also generate the immune response.
  • In addition to the above, lymphoid tissue also occurs as loosely arranged follicles, known as mucosa-associated lymphoid tissue (MALT). They are located within the lining of the respiratory, digestive, and urogenital tract.


Immunity refers to the ability of a host organism to fight disease-causing organisms. Immunity is of two types:

  1. Innate Immunity
  2. Acquired Immunity

Innate Immunity: Innate immunity is already present at the time of birth. It renders effective barriers to the entry of foreign particles and pathogens in the body. It is divided into the following;

  • Physical Barriers: The first and the foremost barrier to the entry of pathogens is provided by the skin. The mucus lining of the respiratory, digestive, and urogenital tract is another powerful barrier and traps the microorganisms entering the human body.
  • Physiological Barriers: The growth of microbes is prevented by the hydrochloric acid of the stomach, saliva of the mouth, and lysozyme in tears of the eyes.
  • Cellular Barriers: Various cells and tissues are also responsible for protecting the human body against diseases. These include neutrophils, natural killer cells, and monocytes present in the blood, and macrophages present in the tissues.
  • Cytokine Barriers: In the case of a viral infection, the healthy cells are protected from the virus by the action of interferons. The interferons are secreted by the virus-infected cells.

Immunity types

Acquired Immunity: The acquired immunity protects the human body from various pathogens. It is much more specific. It is normally inactive and becomes activated only after interaction with specific pathogens. The first encounter with the foreign microorganism is called primary immune response. The subsequent encounters are intense and are referred to as the secondary or anamnestic response. The acquired responses are of two types:

  • Humoral Immunity: The humoral immunity response is carried out by the antibodies. The antibodies are present in the blood and produced by B-lymphocytes. Each antibody molecule has two small light chains and two long heavy chains and is represented as  H2L2. There are five types of antibodies produced by B-lymphocytes: IgA, IgD, IgE, IgG, and IgM.

Structure of antibody

  • Cell-Mediated Immunity: The cell-mediated immunity is moderated by the T-lymphocytes. It does not involve the action of the antibodies.  The cells which are responsible for carrying out cell-mediated immunity are phagocytes, cytotoxic T-lymphocytes, and cytokines.

Cell mediated immunity

Active and Passive Immunity

  • Active Immunity: The antibodies are produced when the host cell encounters an antigen. In the case of active immunity, a host is deliberately exposed to an antigen, which may be living or dead or other proteins. An active response also occurs when a pathogen gains entry into the body during a natural infection.
  • Passive Immunity: The passive immune response occurs when antibodies are administered directly into the body; for protection against pathogens. Colostrum, secreted during early lactation, is a rich source of antibodies.

Active passive immunity

Vaccination and Immunisation


  • Vaccination: In vaccination, weakened/inactivated pathogen or antigens of the pathogens are administered into the human body.
  • Immunisation: In immunisation, a person becomes immune/resistant to a disease by administration of a vaccine.


When the immune system renders an exaggerated response to certain antigens, then that exaggerated response is known as allergy. Allergies are caused by the hypersensitivity of the immune system. Allergens are the substances against which an immune response occurs. The symptoms of an allergy can be reduced by the use of anti-histamine, adrenaline, and steroid drugs.

Immune System: Diseases

Autoimmune Diseases: Autoimmune diseases occur when the immune system of a human starts targeting self-cells. Examples of the autoimmune diseases are:

  • Rheumatoid Arthritis
  • Systemic lupus erythematosus (lupus)
  • Addison’s Disease
  • Grave’s Disease
  • Myasthenia Gravis

Immunodeficiency Disorders: Immunodeficiency disorders occur when the immune system of a human is not as effective as normal, therefore, even mild infections can result in life-threatening conditions. AIDS (Acquired Immuno Deficiency Syndrome) caused by HIV (Human Immunodeficiency Virus) is one of the most important examples of immunodeficiency disorders.

Severe Combined Immunodeficiency (SCID): SCID constitutes rare and congenital disorders with low or insignificant immune response. The condition arises because of the defect in the development of B- and T-lymphocytes.

Lymphoma: Lymphoma refers to the cancer of the lymphocytes; lymphocytes are the cells which fight against the infection.



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