Overview of the Immune System

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Before you study any topic in immunology, it is important to have an overview of the immune system. This overview will serve as a map of the immune system parts. Whenever you have a map of a place, you are unlikely to get lost.

The immune system is divided into two broad categories. That is the innate arm and the adaptive arm of the immune system. Part of the innate is also the complement system considered one of the effector mechanisms of innate immunity. The adaptive arm is further divided into humoral immunity (Mediated by B cells which produce antibodies) and cell-mediated immunity (Mediated by the T-cells).

The innate arm of the immune system has several structures besides the immune cells that help your body to resist infection in the first place. This is not always possible but is possible most of the time. If you cannot resist infection inside your body your immune system is elaborate and it will help you deal with the attacker. 

Overview of the Immune System Components

The immune system is made up of many parts and players. The different parts are well coordinated to make your immune system a perfect art. And it really is. The complexity of the immune system is a result of many types of cells, tissues, molecules, proteins, and other important players. Let’s discuss all the components of the immune system now.

Skin, mucous membranes, Normal Flora, Stomach Acid, and Enzymes

These organs and structures are called first-line defense barriers. They attempt to block the entry of microorganisms into your body. If they do this successfully, the rest of your immune system will have no work to do, isn’t it? But because microorganisms can sometimes cross these barriers, the immune cells, tissues, and organs are involved in the fight later.

  • Skin: The skin is the interface between the outside world and your body’s internal organs. It has layers that make it an effective barrier that prevents microorganisms from entering your body. When the skin is intact, many microorganisms cannot enter your body. However, there are a few which can penetrate the skin like cercariae of schistosomes.
  • Mucous Membranes: The mucous membranes are mostly found lining hollow systems like the gut, urinary tract, respiratory tract, and reproductive organs. The mucous membranes secrete mucous that makes it hard for microorganisms to infect the underlying cells and tissues.
  • Enzymes: Your salivary glands and sweat glands produce fluids that contain enzymes like lysozymes that can kill microorganisms. Other fluids that contain such enzymes include mucus and tears.
  • Stomach acid: In your stomach, there is so much hydrochloric acid (HCL). Besides helping you to digest food, HCL kills many microorganisms when they enter your gut. This means they will not be able to infect you.
  • Normal flora: In your stomach and elsewhere in your body, there are also bacteria called normal flora (beneficial bacteria). These bacteria deprive pathogenic bacteria of important supplies that they need to survive and multiply. This is why they are called ‘good bacteria’.

Primary Lymphoid Organs

These are organs that are involved in the development and maturation of immune cells. The primary lymphoid organs include the bone marrow and the thymus.

Bone marrow

The bone marrow is perhaps the most important primary lymphoid organ. This is where the development of all blood cells begins and where it is completed for some types of immune cells. Some other immune cells like the T lymphocytes migrate to the thymus where they complete their development.

Therefore, the bone marrow contains many types of stem cells that include those destined to become red blood cells which are not immune cells. The bone marrow is contained at the core of your bones throughout the body.


This is an organ that is located mid-sternum in your upper chest. The thymus houses the developing T lymphocytes and nurtures them to maturity. The T cells enter your thymus through its medulla and continue to develop through the cortex before they can exit as mature naïve T cells.

As they leave the thymus to the peripheral circulation, T cells have a fully developed T cell receptor (TCR) that can help them recognize and deal with any foreign antigen. In the thymus, the T cells are also trained on how to distinguish between self and foreign antigens. This helps to ensure that they don’t attack your cells in what is called autoimmunity.

Anyone without a thymus or without a fully functional thymus will not have functional T lymphocytes. Such people will be vulnerable to infections of all types. The reason for this is that the T lymphocytes support many other immune cells in their work in protecting the human body.

Secondary Lymphoid Organs

Lymph nodes

These are important components of your lymphatic system. The lymph node is where the lymphatic vessels drain their lymph fluid which sometimes contains microorganisms and other antigens.

In the lymph nodes, the immune cells also arrive through the blood vessels. It is here that your immune cells interact with antigens before they can replicate in huge numbers. The antigen-specific lymphocytes are then sent back to the blood circulation to clear the antigen.

During the infection, the lymph nodes are swollen because of the accumulation of all these cells and antigens. Once an infection resolves, the swelling subsides, and I’m sure this is an experience you have gone through at some point in your life or even now as we speak.

Lymph nodes are distributed throughout your body. Generally speaking, lymph nodes are named and classified according to where they are found in your body. These categories are inguinal lymph nodes (groin), mediastinal (chest), pelvic (pelvis), axillary (armpits), cervical (neck), and retroperitoneal (back of the abdomen)


The spleen is also an important immune organ. It houses mature B cells as they interact with antigens and nurtures them until they are fully activated B cells during infection. This underscores the importance of the spleen in the humoral immune response. 

The spleen also serves as a filter that traps old, abnormal, and damaged red blood cells, and finally destroy them. The spleen stores the thrombocytes or platelets and breaks them down when they are not needed by your body.


In the same way, as other lymph nodes do throughout the body, tonsils, trap antigens. The immune cells in them recognize and kill the microorganisms. The B cells produce antibodies that help to eliminate specific microorganisms.

When there is an infection of the throat or nearby places, your tonsils will swell for a few days until the infection resolves. When the tonsils swell you can hardly swallow food and it’s certainly a hectic moment for you. The various types of tonsils that exist in your body include adenoids, lingual and palatine tonsils.

Immune Cells

We have two categories of immune cells in your body. The cells of the innate immunity and the cells of the adaptive immunity. We shall do an overview of each of these cells here to understand which cells do what in the immune system. However, before we do, we shall explore the origin of these cells.

Origin of immune cells

All immune cells originate from a common progenitor and begin their development in the bone marrow. We have included a diagram below that shows how different precursor cells commit to the different lineages of development. See below:

Development of immune cells in the bone marrow from the common lymphoid progenitor (Deposit Photos)

Cells of the innate immunity

  • Neutrophils – Specialised in phagocytosis of pathogens particularly bacteria
  • Basophils – Involves in allergic reactions
  • Eosinophils – Involved in fighting parasitic infections
  • Natural Killer Cells – Resemble the lymphocytes and help to fight viruses and tumors
  • Mast Cells – Involved in hypersensitivity reactions by degranulating
  • Dendritic Cells – Involved in phagocytosis and antigen presentation to T cells
  • MacrophagesInvolved in phagocytosis and antigen presentation to T cells
  • Monocytes – Precursors of dendritic cells, macrophages, and mast cells

Cells of the adaptive immunity

  • T-lymphocytes – These cells include the T-helper cells (CD4+), the cytotoxic T cells (CD8+), T regulatory cells (CD4+, CD25+, FoxP3+)
  • B – lymphocytes – These cells change to plasma cells after encountering an antigen and plasma cells secrete antibodies


The immune system is a complex system that is made of many components. The components are well coordinated to achieve one goal. That goal is the elimination of the foreign antigen. Firstly, we have the first line of defense. If defeated, then we have several other components that are involved in protecting you.

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25 thoughts on “Overview of the Immune System

  1. Mr Ochieng N says:

    So simple and helpful .But I can’t spot functions of T regulatory cells ,they are only mentioned.

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