How the Immune System Works

This information is intended to provide you with a general understanding of the cells of the immune system and the role they play in fighting infections.

The immune system is a complex network of cells that work together to protect the body against “foreign” or invading cells, including abnormal cells that can lead to cancer.

Most foreign invaders are germs — infection-causing organisms that you may come into contact with, such as bacteria, viruses, and fungi. The immune cells are constantly on guard and ready to go into action to detect and destroy these invaders.

The immune system can recognize millions of different enemies and produce secretions and cells to wipe out each of them.

The secret to the immune system’s success is its complex and exciting communications network. Tens of millions of cells, organized into groups, pass information back and forth, much like bees do when they swarm. When one group of immune cells discovers the presence of foreign invaders, it undergoes key changes and produces powerful chemical substances. These substances allow the cells to regulate their own growth and function, enlist other immune cells, and direct them to attack the foreign invaders as well.

How Immune Cells Distinguish Between Foreign Cells and Normal Cells

The immune system has an amazing ability to distinguish between the body’s own cells (self) and foreign cells (non-self).

Each cell carries protein markers called antigens that identify it as self or non-self.

The body’s immune system co-exists peacefully with all cells that carry the self-marker.

Non-self markers are found on all foreign cells including bacteria, fungi, viruses, transplanted tissues or cells from another person and abnormal cells that can lead to cancer.

Key Immune System Cells and Their Functions

All immune cells are white blood cells (WBCs) that arise from a single pluripotent stem cell, which is found in bone marrow.

When a pluripotent stem cell divides, it forms two daughter cells which either remain pluripotent stem cells to replenish the pluripotent stem cell pool, or they become specialized immune cells with dedicated functions. Some of these immune cells will become part of the myeloid lineage, which generates cells such as neutrophils, monocytes/macrophages, and dendritic cells. Others will become part of the lymphoid lineage, which gives rise to T-cells and B-cells.

The immune system stockpiles a huge arsenal of cells. Some immune cells can destroy many foreign invaders, while others are activated to attack and destroy only specific targets.

In order for the immune system to work effectively, the immune cells must not only communicate, but also cooperate with each other.

This information will identify the key cells of the immune system and describe how they work together to fight infection.

Immune Cell Lineage



2Neutrophils are the most abundant type of WBC and are the first immune system cells to rush to the site of an infection.

Neutrophils are cell eaters, they engulf foreign organisms and use pre-packaged chemicals to destroy them.

Although neutrophils are rapidly produced by the immune system, they live and work for a relatively short period of time — usually just a few days.

Neutrophils are destroyed during chemotherapy for leukaemia resulting in the Neutropaenic Phase where the patient is extremely susceptible to infection.


Monocytes and macrophages

34Monocytes and macrophages are related to neutrophils and are also cell eaters, they destroy foreign organisms by ingesting them.

Monocytes are produced by the bone marrow and released into the bloodstream.

Some monocytes enter the tissues, then enlarge and mature into macrophages, or “big eaters.”

Macrophages play many important roles within the immune system. As scavengers, they rid the body of worn-out cells and other debris.

They also secrete a wide range of powerful, antigen-destroying chemicals and work to “activate” other immune cells through a process called antigen presentation (which is described later).

Although they are not as abundant as neutrophils, Monocytes and Macrophages can recognize and destroy more types of foreign invaders, including viruses.

Also, while neutrophils have a very short lifespan, macrophages can remain in the tissues for months or even years. They act as lookouts, watching for invaders.

When necessary they can also direct other immune cells, such as neutrophils, to the site of an infection.


Dendritic Cells

5Although dendritic cells are the least abundant type of WBC, they are the most potent, and are found in the blood and in most tissues throughout the body. Dendritic cells are regulators or “coaches” of the immune system. They act as observers and messengers who go to the battle site, collect pertinent information, and pass it along.

Dendritic cells have long, finger-like projections that increase their size, improve their ability to move around, and increase their ability to find and sample more antigens.

They collect information about the infection or invasion by identifying the type and the location of the invader.

Dendritic cells are unique because they are capable of capturing and absorbing many types of foreign antigens.

Once they capture the antigens, dendritic cells migrate to the lymph nodes, where they use the captured antigens to “activate” large numbers of T-cells through antigen presentation. This activation process tells the T-cells how to respond.

Thus, dendritic cells dictate how other parts of the immune system should react.

Dendritic cells are often called “professional” antigen-presenting cells because they do it better than any other type of immune cell.


In order for T-cells to do their job they must be “activated” by other immune cells, such as macrophages and dendritic cells.

Upon activation, T-cells mature to become either:

  1. Helper T-cells which direct and regulate the body’s immune responses
  2. Killer T-cells which directly attack cells that

are infected or cancerous.


B-cells produce antibodies, which are proteins that bind to antigens and mark them for destruction. B-cells are activated when a   T-cell presents an antigen to them. The B-cells then begin producing antibodies that bind specifically to that particular antigen, which is similar to the way a key matches a lock. Activated B-cells also begin multiplying, which further increases the body’s immune response.


How the Immune System Responds to Infection

Antigen, T-cell, B-cell, Antibody

Once foreign cells enter the body, the immune system fights them using two armies of cells:

  1. nonspecific cellsNeutrophils, macrophages, and dendritic cells which attack foreign invaders of all kinds and circulate continuously within the body looking for foreign antigens

This line of protection is known as Innate Immunity

  1. specific cells- T- cells and B –cells

which only go into action when the nonspecific cells  give them information about new invaders.

When macrophages and dendritic cells attack and destroy a foreign invader, they capture a portion of it and then present it to the body’s T-cells.

These T-cells then aggressively seek out and destroy all foreign invaders that carry that specific antigen.

T-cells can also activate B-cells as described above.

This line of protection is called Adaptive Immunity

It is through these and other complex processes that the body’s immune cells work together to destroy foreign invaders and successfully fight off infection.


Information and diagrams sourced from, editing by JRSH, formatting by LHH