Immune system: A complete Guide

 What is the immune system?

The immune system is your body's defense against infections, viruses, and other potentially harmful substances. It allows you to fight infections by killing bacteria, fungi, or virus-infected cells in your body without causing any damage to the healthy ones. The immune system also identifies cancerous cells before they spread through the rest of the body so that it can kill them as well.

This is the system that enables your body to produce antibodies against bacteria and viruses, which will make you immune from these infections in case of a re-infection. This process generally takes up to two weeks or more before it becomes effective for some types of infections. 

immune system
immune system

The second method through which the immune system works is by killing off any infected cells. This may be a faster method of killing the infection, but it can lead to tissue damage as well because your immune system doesn't discriminate between healthy and harmful cells in some cases.

The immune system has two parts: innate immunity and adaptive or acquired immunity. The former is present from birth while the latter develops through time after exposure to antigens. Acquired immunity can be further divided into specific or non-specific immunity, depending on how it recognizes the antigens that are harmful to your body.

Parts of the immune system.

The human immune system is made up of a complex network that protects the body from infections and other foreign invaders. The two major components of this system are:

white blood cells

White blood cells, also known as leukocytes, are a type of cell that fights against infection. They’re part of the immune system and play an important role in protecting the body from disease-causing organisms, such as bacteria or viruses. White blood cells help fight infections by killing germs and destroying harmful substances like toxins.

The most common white blood cells are neutrophils, eosinophils, basophils, and lymphocytes. They’re found in the body's bloodstream or circulate through tissue spaces outside of your circulation. The neutrophil is the main type of white blood cell that fights infection by destroying bacteria. 

Eosinophil is a type of white blood cell that acts as an antigen and helps your body fight off parasites. Basophil is another kind of white blood cell, which releases histamines to try to destroy harmful substances in the body. 


Antibodies are proteins that your body uses to fight infections. They also help keep you healthy by stopping bacteria and viruses from entering the cells of your immune system. Antibodies can do this because they recognize these foreign invaders and bind to them, which tells other parts of the immune system it's time for action. The job of an antibody is to protect by binding to invaders and trapping them in the body.

Plus, antibodies are very specific. 

Another thing that makes antibodies great is that they are very flexible. 

They can be used to neutralize toxins, 

Create vaccines that teach your body how to fight infection. 

Antibodies are also key in fighting cancers and autoimmune diseases because they have the ability to inform immune cells of their presence. This then helps other components of the immune system destroy them before any damage is done. 

Complement system

The complement system is a part of the immune response.

It helps to fight against infectious agents, damaged cells, and harmful blood clots. Complements are proteins that circulate in body fluids bound to white blood cells or plasma components. There are three different complements with several subgroups based on their biochemical properties: the classical complement pathway, the alternative pathway, lectin pathways.

The classical complement pathway is a protective mechanism that occurs in all mammals and humans as well. 

It involves three different types of proteins: C0-Cascade proteases, C' enzymes, and membrane attack complex (MAC). 

Those components work together to protect cells from infectious agents or foreign substances such as viruses, bacteria, or fungi.

The alternative pathway is an alternative protective mechanism that is activated when the classical pathway becomes non-functional. It starts with spontaneous hydrolysis of the C' component, followed by the creation of MAC and the inflammation process.

the lectin pathway is a part of the innate immune system and is used to fight against fungal infections.

lymphatic system

The lymphatic system is a unique system of vessels and organs that forms part of the circulatory system. It is known as an immune defense network, which transports fluid between tissues and blood vessels back to the heart. The primary role within this transport process involves collecting excess tissue fluids from interstitial spaces in various parts of the body where it returns to the blood vessels.

Lymphatic system organs include lymph nodes, spleen, thymus, and tonsils. It also includes lymphoid tissues that are present in the digestive tract (Peyer's patches), lungs (the respiratory zone of the alveoli), bones (bone marrow), and other parts of our body like inside our cheeks and throat.

The lymphatic system is a vital part of the immune defense process because it allows for fluid from tissues to flow back into blood vessels, which in turn prevents tissue from becoming dehydrated or swelled. In addition, white blood cells are able to move easily through all areas of this network on their way toward infection sites or other damaged parts.


The spleen is an organ that filters blood, destroying old red blood cells and producing new ones. It is located in the upper left side of the abdomen right under your rib cage. The spleen can also help fight infection by trapping certain bacteria or fungi inside its connective tissue walls before they enter the bloodstream. A ruptured spleen can be fatal because it can prevent the body from filtering blood properly, leaving harmful components in your system.

The spleen is: located in the upper left side of the abdomen right under your rib cage and helps fight infection by trapping certain bacteria or fungi inside its walls before they enter into the bloodstream. A ruptured spleen can be fatal because it can prevent the body from filtering blood properly, leaving harmful components in your system.

Bone marrow

Bone marrow is a spongy tissue found in the center of most large bones. In adults, the bone marrow makes red blood cells and platelets that help your body fight infections and stop bleeding. It also produces white blood cells that help you resist infection. Additionally, bone marrow is where most of the body's blood cells are made. Defects in bone marrow can cause a variety of health problems. Some of these are anemia, leukemia, and other blood diseases.


The thymus gland is a part of the immune system that produces T cells. These are white blood cells that fight off bacteria, viruses, and other germs. The function of this gland decreases during puberty and continues to decrease with age into adulthood. It normally becomes less active after early childhood but can start working again in response to immunization.

In the thymus, T cells become "educated" about what is best for your body and how to defend it from harm. They are taught: not to attack proteins in their own bodies (called self-tolerance), only to destroy germs and other invaders (called non-self). But sometimes these educated T cells begin attacking the body's proteins (self-tolerance). When this happens, the disease is called an autoimmune disorder.

The thymus gland works best at a high altitude and becomes less effective as we live in lower altitudes. This might be one reason why people who live above 3000 feet have a much greater risk of developing tuberculosis than those living below that.

What is immunity?

The human body has developed a number of ways to protect itself against foreign invaders like viruses and bacteria. Immunology, the study of how our bodies fight infection, is an incredible field that’s led to some truly remarkable breakthroughs in medicine—and it all begins with one keyword: immunity. Immunity is a broad term that refers to the millions of different ways our bodies defend against infection and disease.

Immunity can be broken down into two types: innate immunity and adaptive or acquired immunity. 

Innate immunity

Innate immunity is the body's first line of defense against pathogens. It helps protect us even without being exposed to a pathogen, which makes it very effective at protecting us from infectious diseases. Innate immunity does not require previous exposure or infection for successful protection. This is in contrast with acquired/adaptive immunity, where an individual must be exposed to a pathogen before immunity can develop. 

There are several types of cells that function as part of our innate immunity, and they include phagocytes, natural killer cells (NK), epithelial cells, mast cells, complement proteins, interferons, and coagulation factors. Each of these plays a critical role in protecting our bodies from pathogens, but there are key differences between them all.

Phagocytes play a central role in innate immunity because they eat up foreign invaders and digest them so that T cells can kill any leftover pieces when phagocytes die. Natural killer (NK) cells also work to kill pathogens, but they do not require phagocytes to be successful. In fact, NK cells can even target tumor cells and other infected human cells without having prior exposure or infection.

Epithelial cells line our skin, mouth, throat, and intestines in order to keep microorganisms from entering our body through those surfaces. Mast Cells are responsible for releasing histamines, which are chemicals that help control inflammation. Histamine is released when mast cells come into contact with pathogens. Complement proteins work with antibodies to destroy viruses and bacteria by punching holes in their cell membranes.

Interferons also attack pathogenic invaders but instead of making direct contact, they release signaling molecules that activate other parts of our innate immune system. The body's coagulation factors work to limit blood loss by helping the formation of clots that stop bleeding.

Acquired immunity

The acquired immunity is a special type of secondary immune response, where the immune system adapts to antigens that it has previously encountered. It occurs when antibodies and lymphocytes are produced by an individual after being exposed to a pathogen or antigen for the first time. This process often results in them having developed long-term resistance against certain pathogens after only a single exposure.

this means that the immune system is able to recognize and remember specific antigens. This allows it to quickly respond when exposed again in the future, thereby helping protect against disease or infection.

The immune system and microbial infection.

The immune system is made up of a number of different cells and proteins. These work together to attack pathogens such as bacteria, parasites, fungi, and viruses that cause an infection. They get rid of the pathogen before it causes damage or spreads further. This means that we usually don’t realize we have been infected by something until it has gone.  In fact, it can take several days for symptoms of infection to show.

When the microbes bypass the first line of defense and enter the body they need to get past another level - called the second line of defense. This is made up of cells such as macrophages and neutrophils, which circulate in the bloodstream waiting for the infection to occur.

After passing all the lines of defenses, the pathogen is now in the body and can cause infection. the severity of infection depends upon the causing agent.

How an immune response works?

The cells of the immune system recognize a foreign antigen through special receptors that are located on their membranes. When an invader is detected, some important changes take place in these cells: they divide and secrete proteins called antibodies, which bind to the antigens on the invaders. These antibodies mark them for destruction by other lymphocytes or phagocytic cells.

As for the body's response to infectious organisms, it is interesting that some bacterial diseases are treated with antibiotics, which kill bacteria by inhibiting their growth or destroying them directly. However, this treatment does not remove the antigen from the host because most of these antigens are intracellular. To overcome this problem, lymphocytes and phagocytic cells can destroy intracellular bacteria.

The body's response to infectious organisms is also affected by the fact that viruses are obligate intracellular parasites, which means they can multiply only inside living host cells. Since most of these antigens exist within infected cells, lymphocytes and phagocytic cells are not able to destroy them. To overcome this problem, the body responds with a specific immune response.

How do antibiotics help fight infections?

Antibiotics are drugs that kill or stop the growth of bacteria. Bacteria live in your body and they're usually harmless, but sometimes they can cause an infection. An antibiotic fights against bacterial infections by either killing the bacteria or stopping them from multiplying so that our bodies natural defenses (such as white blood cells) can destroy them.

-A doctor can prescribe an antibiotic if you have a bacterial infection. 

-Before taking antibiotics, make sure that it is prescribed for your illness and that you are not allergic to any of the ingredients. 

-Do not take antibiotics unless they have been prescribed by your health care provider because there are some types of infections that cannot be cured with them.

-If you have a viral infection, antibiotics will not work on it and can even make your illness worse. 

-Some bacterial infections may be resistant to certain types of antibiotics which is why some doctors prescribe more than one type at the same time. If this doesn't help then there might be another reason that has caused the antibiotic resistance.

-Antibiotics may have side effects such as upset stomach, vomiting, or diarrhea. If you experience any of these symptoms then contact your doctor right away. 

-The most common types of antibiotics are penicillin, amoxicillin, tetracycline,  cephalosporin, sulfonamide, streptomycin.

-If you have taken an antibiotic for your illness and it doesn't seem to be working then talk to a doctor because you might have developed resistance. 

-Be sure that the course of antibiotics is finished even if you are feeling better because stopping treatment early can cause the infection to come back.

-Antibiotics are very helpful when used correctly, but if you stop taking them before they have finished then it can make your illness worse in the future. 

-Do not take antibiotics prescribed for someone else or that were bought over the counter because this is dangerous and can lead to antibiotic resistance.

Other body's defenses against microbes

Other body defenses against microbes include skin and mucous membranes such as those in the nose, lungs, and gut. 

In addition to these passive defenses, our bodies have active ones that can combat pathogens by attacking them directly or calling for help from other cells. Our immune system is a complex collection of organs with many types of defensive cells.


 It is usually the result of an infection in your body. Fevers are typically caused by infections, but sometimes they can also be a symptom of other problems that you have with your health or physical condition. It is important to understand what's causing this fever so that proper measures can be taken if necessary. If you're not sure why you have a fever, it is advised to seek out help from a doctor.

A person may feel cold and shivery at first as their body temperature begins to rise. They might also experience headaches, chills, muscle aches, and fatigue that go along with this illness. As the fever gets higher you'll start feeling hot and sweaty and will probably need to take off some of your clothes. You'll also have a high heart rate and be flushed with this condition as well.

An individual might notice that they are sleeping more than usual when they have a fever due to how lethargic it makes them feel. It's important to get enough rest while you're sick so your body can recover from the infection. But, if you are unable to get any sleep because of the fever that is not a problem either since it can be helpful in fighting off an illness.

People with fevers need plenty of liquids and some light food while they're sick as well. It's important for them not to eat greasy or heavy foods though when their stomach is upset.

What are Immunizations?

The body’s immune system protects it from infections caused by bacteria, viruses, and other germs. But sometimes the body is not able to fight off some infections or diseases because its defenses are weak or may be absent completely. Immunization (vaccination) helps people stay healthy by helping their bodies build a stronger defense against specific diseases. Vaccines stimulate a person’s immune system to produce antibodies that fight against specific diseases.

Vaccines works by:

- Preventing a person from getting the disease. 

- Reducing the severity of symptoms if a person gets sick. 

- Making it less likely for an infected person to spread the infection to other people.

It is important that you get your vaccines on time because many diseases can be prevented by immunization, and some vaccine-preventable diseases can be very serious, even fatal.

Some of the common vaccines are :

- vaccines for measles, mumps, and rubella 

- Influenza vaccine 

- vaccines for shingles. 

-vaccine for TB

-vaccine for polio

What is immunologic memory?

The first time the immune system encounters a pathogen, it can take days to generate an effective response. This is because there are only limited numbers of specialized cells that have receptors for recognizing different pathogens. Once these cells detect something they do not recognize (i.e., antigen), production begins on both antibodies and lymphocytes specific to this pathogen.

However, when the immune system is exposed to an antigen again after this initial response it will respond much faster and more powerfully than the first time around because memory B cells are present in high numbers which can produce antibodies rapidly using their genetic material that has already been programmed for antibody production. These memory lymphocytes also reside long-term in areas of the body that are prone to re-encountering these pathogens.

If an antigen is detected again, memories cells can respond within hours or days depending on the type of lymphocyte which also decides how quickly antibodies will be produced. These memory B and T cells protect against future infection by either neutralizing antigens directly (B cell) or by activating other cells of the immune system (T cell).

It is important to note that immunologic memory only provides protection against future infection with the same pathogen. Immunological memory does not provide immunity against all pathogens and it takes time for this response to develop, so during this time an individual can become infected if exposed again. 

disorders of the immune system

An immune system disorder is any problem or disease that occurs when your body's immune system fails to work properly. Immune disorders are caused by either a lack of healthy cells, called immunodeficiency, or an overactive defense mechanism resulting in autoimmune diseases. 

Some of the immune disorders are:

- Anemia. 

- Autoimmune diseases, including rheumatoid arthritis and lupus. 

- Cancer. 

- Chronic fatigue syndrome (CFS). 

- Diabetes Type I/insulin dependent diabetes mellitus (IDDM) / juvenile diabetes.

- Epstein Barr Virus infection or chronic mononucleosis.


- Hypogammaglobulinemia or reduced levels of antibodies in the bloodstream, which makes it difficult for your body to fight infections and certain diseases that have a vaccine available.

- Immunodeficiency disorder, including common variable immunodeficiency (CVID).  

- Pneumonia and other respiratory infections, pneumonia is a common infection in people whose immune system isn't working properly or who have a weak immune system.

- Rheumatoid arthritis.

- Tuberculosis (TB). 

- Viral hepatitis B and C, which affects the liver. 

- Systemic lupus erythematosus (SLE) or lupus.

- Viral infections, including herpes simplex virus type I and II.

- Warts are caused by the human papillomavirus (HPV). 

Immunoglobulin therapy

 It is an effective treatment for many people with primary immunodeficiency diseases. Under normal circumstances, lymphocytes are activated by the antigen-presenting cells of the immune system to mature and differentiate into T helper cells that produce cytokines that activate cytotoxic T cells or antibody-producing B cells. With higher rates of infection in these patients, it is helpful to treat with immunoglobulin by infusing antibodies specific for microbes that are causing the infection.