Understanding The Immune System

Understanding The Immune System:

Components of Immune System

Our immune system, made of a network of cells, tissues, and organs, work together to protect us when micro-organisms invade the body. The system can distinguish what belongs to the body from everything foreign to it, and  protect it against infections by foreign substances.
Let’s say a microorganism breaches one of your natural defenses (i.e., skin, mucosal surfaces, etc.), and you just received a cut on your finger. Your body now faces the risk of a full-scale invasion from bacteria or viruses. Let’s see how your body protects you from such a threat.
The cells at the site of injury have been destroyed by the cut and the body responds by dilating the blood vessels and increasing the blood flow. These cells also alert other cells of the immune system of the impending danger and promote migration of other cells.
This brings the immune system’s primary defensive immune cells – the white blood cells or leukocytes – to the damaged area in great numbers.
Leukocytes come in two basic types: phagocytes and lymphocytes. Phagocytes initiate search-and-destroy attack on invading organisms. Neutrophils are phagocytes which phagocytize foreign material. They contain granules filled with potent chemicals capable of destroying microorganisms.

Monocytes are phagocytes which circulate in the blood, then migrate into tissues where they develop into macrophages (”big eaters”). They find and “swallow” bacteria, viruses, and dead or injured body cells and throw worn-out cells and debris out of the body.  

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Lymphocytes: Producing Antibodies Against Antigens

Foreign substances are capable of triggering an immune response in the body and are called antigens.
Antigens are proteins that may be contained within or on bacteria, viruses, other microorganisms, or cancer cells. Antigens may also exist independently—for example, as food molecules or pollen. Bacteria and viruses have antigens on their surface which are different from the body’s “self” antigens.
A normal immune response consists of recognizing the potentially harmful pathogenic antigens, activating and mobilizing forces to attack it.
Lymphocytes are one of the chief components of the immune system which is responsible for this. They are formed in lymphatic tissue throughout the body including the spleen, thymus, tonsils, and lymph nodes.
There are two types of lymphocytes. B lymphocytes are the body’s military intelligence system: they identify the invaders, and T cells destroy the invaders which the B cells have identified.
B Cells
About 10% of lymphocytes that circulate in the blood are B cells. On the surface of each lymphatic cell are receptors that enable them to recognize foreign substances and alert the rest of the immune system about the presence of invaders. The two types B cell and T cells differ in function and the molecules that are on their surface. These ‘recognition molecules’ allow immune cells such as B cells, T cells, and macrophages to recognize and communicate with one another and determine which antigens an individual can respond to.
When they detect antigens, the B cells set out in search for antigen matching its receptors. When they spot the right antigen, they become activated and travel to the spleen or the lymph nodes. With the help of helper T cells, the B cell starts to divide to produce clones of itself. During this process, two new cell types are created, plasma cells and B memory cells.
The plasma cell is specialized in producing a specific protein, called an antibody, that will respond to the same antigen that matched the B cell receptor. Antibodies are released from the plasma cell so that they can seek out intruders and help destroy them. Plasma cells produce antibodies at an amazing rate and release tens of thousands of antibodies per second.
Antibodies  are special Y-shaped proteins that respond to a specific antigen and attaches to it like a key with a lock. This way, they act as receptors for antigens and mark intruders for destruction by other cells. 
Each branch of the Y-shaped antibody can bind to a different antigen, so while one branch binds to an antigen on one cell, the other branch could bind to another cell – in this way pathogens are gathered into larger groups that are easier for phagocyte cells to devour.
Recognition of an antigen by an antibody ”tags” it for attack by other parts of the immune system. Cells, or molecules that are marked with antibodies are clumped together and phagocytized (engulfed) by neutrophils or macrophages.
Antibodies also neutralize toxins and disable viruses, preventing them from infecting new cells.
Though the receptors of each lymphocyte cell can only match one specific type of antigen, the body produces so many different lymphocyte cells that the immune system can recognize nearly all invaders.
As said previously, the division of B cells also produces Memory Cells. These cells have a prolonged life span and can thereby “remember” specific intruders. T cells can also produce memory cells with an even longer life span than B memory cells. The second time an intruder tries to invade the body, B and T memory cells help the immune system to activate much faster.
T Cells
T cells start in the marrow but migrate through the bloodstream to the thymus, located in the upper part of the chest and mature there. When they are exposed to antigens, they rapidly divide and produce large numbers of new T cells that are sensitive to that type of antigen. More than 80% of lymphocytes in the circulating blood are T cells.
Antigen Presentation
After a macrophage eats an invader, it travels to the nearest lymph node and displays an antigen fragment from the invader on its own surface, to present information about the captured pathogen. Antigen presentation stimulates T cells to become either cytotoxic (”killer”) cells or “helper” cells.
When the receptor of a helper T cell recognizes the antigen, the T cell is activated. Once activated, helper T cells start to divide and to produce proteins that activate B and T cells as well as other immune cells.

Credits: Nobelprize.org

Killer T Cells

The killer T cells produce chemical substances known as lymphokines that are essential in helping the B cells attack cells of the body infected by viruses, bacteria or even cancer cells. The  receptors of killer T cell searches each cell that it meets and kills it if it finds it infected with antigens.

Image via Wikipedia
Cytotoxic T-cells induce the death of infected somatic cells or tumor cells.  These are T cells that are capable of inducing the death of tumor cells or those infected with viruses, or are otherwise damaged or dysfunctional.

Image via Wikipedia

Cancer and Natural Killer Cells

Natural Killer Cells are the most aggressive white cells in the immune system, and constitute about 5% to 15% of the total lymphocytes. NK cells are part of the first line of cellular defence and they are involved in our body’s natural immune response. They target tumor cell and protect against cancer.
They were named “natural killers” because of the initial notion that they do not require activation in order to kill cells that are missing “self” markers.
NK cells attack foreign cells without first having to recognize specific antigens like the killer T cells. NK cells identify specific cell types (tumour, infection or foreign to the body) and destroy them. These cells play an important role in the rejection of xenotransplantation (transplantation from another species).


Researchers at the University of Maastricht have succeeded in curing breast cancer in mice by injecting them with natural killer cells or NK cells. NK cells attack the tumours without attacking any other cells in the body.

Auto-immune diseases: Confusion over self and non-self

Before lymphocytes join the fight against infection, they must first learn the difference between self and non-self. The body is able to detect irregularities in the B and T lymphocytes and destroys them even before releasing them into blood circulation. When this screening by the body fails, the defective cells mistakenly believes body’s own tissue as as “foreign,” and tries to destroy it.
Diabetes and rheumatic arthritis are a few of the ailments that are the result of inappropriate autoimmune response. In the case of Diabetes, the immune system attacks the cells in the pancreas, whereas in the case of rheumatic arthritis, the body attacks the cells in the joints. Allergies are really just the immune system overreacting to certain stimuli that other people don’t react to at all.