The air we breathe, the water we drink, and the food we eat contain a large number of organisms that do not cause harm. However, there are other microorganisms that cause diseases and represent a real threat to the body. They enter the body for a shelter to live and reproduce.

What happens when our bodies are exposed to such intruding creatures?

The Immune System

The immune system specializes in protecting the body from pathogens, combating pathogens, and eliminating cancerous cells and cells infected with viruses. It consists of a set of physical and chemical barriers and white blood cells (specialized and non-specialized) that can rid the body of pathogens, dismantle them, and prevent their proliferation.

The human immune response is divided into two main types, namely natural (non-specialized) and acquired immunity (specialized).

Natural immunity (non-specialized): The main task of this type of immunity is to prevent pathogens from entering the body or eliminate them as soon as they enter or get rid of infected cells, and this immunity is not specialized (because it does not target a specific type of pathogen).

Natural immunity includes the following (first line of defense):

The Skin

The skin represents an important physical barrier that prevents the entry of pathogens, and the sweat secreted from the skin causes a decrease in the pH level, which reduces the growth of many bacteria on the skin surface.

Mucous Membranes

The mucous membranes secreted from the membranes lining the respiratory tract, gastrointestinal tract, urinary system, and genital system prevent viruses from entering the body.


Tears, saliva, and ear gum are a barrier that prevents pathogens from entering the body because they contain enzymes that break down foreign bodies as well as the hydrochloride acid in the stomach that digests many of the pathogens present in food and drink.

Useful bacteria

There are some types of bacteria that are not harmful to the human body (such as those found in the small and large intestine) that produce some of the acids due to the process of its analysis of foodstuffs that reduce the pH and make the surrounding medium not reproducible to some parasites and bacteria microbes.

But what happens if the first line of defense is breached?

The second Line of Defense

The second line of defense addresses it, as this line consists of immune cells that are not specialized and preventive proteins. If pathogens enter the body, the immune cells defend it.

The second line of defense (composition)

Firstly :

Natural Killer Cells (NK Cells)

Lymphocytes are found in the spleen, lymph nodes, bone marrow, lungs, and blood. They are voracious cells that ingest bacterial pathogens.  They can identify and kill virus-infected cells.


Secondly :

Large Macrophages

Large macroscopic cells. These are mostly cells with a single nucleus and are usually free-moving, wandering from one tissue to another or residing in the organs such as the spleen, liver, tonsils, and lymph nodes, which is specialized in captivating the antigen and decompose it.

macrophage devours an antigen

Thirdly :

The Confrontation Between White Blood Cells and Antigens

A) Natural killer cells (not specialized)

Natural killer cells secrete a substance called perforins that causes holes in the membrane of the affected cell. Then these cells secrete granular enzymes that enter through the holes. They break down the infected cell proteins causing it to die, then a large macrophage cell devours the dead cell.

B) Interferon

They are proteins secreted by virus-infected cells that bind to neighboring cells and stimulate them to produce anti-viral proteins that prevent weakening numbers of viruses attacking them.

C) Inflammation.

The chemicals produced by the pathogens or the affected cells attract the immune cells to the injury area and increase the blood flow towards it in addition to the permeability of the membrane of the capillaries in the area of ​​the injury, which helps to increase the numbers of white blood cells in that area, for the inflammatory response. Symptoms, including redness due to the expansion of the capillaries and swelling, causes the plasma to come out of the blood and the response helps the sensation of pain as a result of nervous infections and high temperature affected tissue.

The surfaces of all cells of the body contain glycoproteins. The function of these proteins is for our body to distinguish between a normal and abnormal body cell from each other. In simple words, imagine like two teams, one is wearing blue, and the other team is wearing red.

The Third Line of Defense

The third line of defense consists of specialized immune cells like :

  • Helper T cell
  • B cells
  • Big macrophages

What Happens When an Antigen Penetrates the Second Line of Defense?

The primary cell responsible for managing this matter is the T helper cell and, in some cases, the big macrophages.

When the second defense line is breached, the helper T cells bind to the antigen, secreting chemicals called cytokines; in some cases, the big macrophages devour the antigen and binds with the helper T cell to "give" it the flag of the antigen so the immune system recognizes it.

When secreting cytokines, the T-helper cells are divided and multiplied into two types of helper T cells, the active helper T-cells and the memory helper T cells. The active helper T-cells activate the natural killer T cells to eliminate the antigen using perforins as mentioned before.

T-cell attacking cancer cell

The cytokines secreted by the active helper T cells stimulate B cells, which in turn activated and divided into two types of cells: plasma B cells, memory B cells. In simple words, the helper T cell is like an undercover agent who is looking for a spy and the cytokines are the alarm siren to defend the body

B cells

(Plasma B cell)

As mentioned before, the cytokines activate the plasma B cell to multiply and divide into two kinds of B cells, plasma, and memory B cells.

The primary duty for plasma B cell is producing antibodies to take out the antigen, while the B memory cell is memorizing the way to handle the situation if the same kind of antigen ever came again and not repeating the recognition cycle again.

There are many ways that the antibiotic produced by the plasma B cell engages the antigen like:

  • Antibodies are bound in the plasma membrane of the cell that causes the disease, which leads to the activation of proteins that create holes in the plasma membrane of the cell that causes the disease and the entry of body fluids into it, so the cell decomposes.
  • The antibody is bound to antigens, causing sedimentation, activating macrophages then devoured.
  • The antibody is bound to the antigen, preventing it from binding to and damaging the cells of the body.

In the end

Our immune system protects us from many pathogens, viruses, and microbes, where we learned about the role of each of the cells that make up this system, and we learned the difference between the specialized immune system and the non-specialized immune system. This process is very brilliant and accurate and worth knowing about.

Be safe .