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What is the human immune system?

The human body is complex with multiple cells, systems, and organs working together to maintain homeostasis. Many environmental and internal factors affect this complex system which can lead to illness of any organ or system. Therefore, one of the important components of this complicated set-up is the immune system which acts as the defense mechanism of the body. Many special cells and organs make up this immune system that both identifies foreign bodies like viruses, bacteria, and other pathogens as well as eliminates them from the body. The sophisticated immune system in the human body is not only equipped to identify various pathogens but is also able to evolve and improve its efficiency by forming immunological memory at the time of the initial attack. This way the body can be prepared and respond more quickly in case of subsequent infection.

The defense system in our body has multiple levels of protection ranging from physical barriers to more specific cells and chemicals that work to prevent or eradicate the pathogen. Unfortunately, a person is not born with a fully developed immune system and therefore requires passive immunity in the form of antibodies passed from their mothers from the placenta or breast milk.

Based on its increasing specificity, the immune system has been divided into two subdivisions: the innate and the adaptive immune system. The innate is present in every individual at birth and consists of both physical barriers as well as chemicals and enzymes that are nonspecific, prevent pathogens from entering the body, and also work to kill them. The skin, gastric acid, cough and sneeze reflex, tears, and urine all work to prevent the entry of microbes inside the body. The leukocytes or WBCs including phagocytes, lymphoid cells, mast cells, eosinophils, basophils, and natural killer cells constitute the cellular component of the innate immune system. These cells not only recognize and kill the microbes but also help in the activation of the adaptive immune system. Inflammation is another important component of the innate immune system that helps in mobilizing the innate immune cells and enhances recovery.

The second subdivision is known as the adaptive immune system due to the fact that it adapts and creates an immunological memory of the pathogen which later helps the body produce a quicker and more customized response in case the same pathogen attacks again. This system is activated by antigens, is conditioned to distinguish non-self from self-antigens, and can initiate a more specific response than the innate system. The adaptive immune system is composed of B and T lymphocytes which work by humoral mechanism and cell-mediated mechanism respectively. The B lymphocytes defend the body by acting as antigen-presenting cells, secreting cytokines, and producing antibodies to fight the microbes. The T-lymphocytes, on the other hand, are further divided into two main types:

Killer T cells that, as the name implies, identify and kill those cells of the body which are infected or dysfunctional.

Helper T cells act as decision-makers for the kind of immune system, innate or adaptive, that needs to be activated for the specific pathogen. They mediate the reaction of these immune systems but do not directly help in killing the infected cells.

Regulatory T cells regulate the immune response to an antigen which is necessary to prevent the hyperactivity of these systems and prevent them from attacking self-antigens.

These cells and the organs that produce them all work together to provide the body with an efficient and functional defense system.

Why do we need a healthy immune system?

A well functioning immune system works 24/7 to protect the body from the countless pathogens and microbes it comes in contact with every day and without it, the risk of infection is much higher. Furthermore, infections in an immunocompromised person are much harder to treat and recur more frequently than in immunocompetent persons. A weak immune system can be congenital, due to environmental factors or even as a consequence of certain medications. An immunocompromised person commonly suffers from recurrent infections, especially involving the lungs and the skin, and these people may also complain of gastrointestinal symptoms, cramps, and generalized weakness. This may be especially worrisome in children as it can lead to a delay in the growth and development of that child.

Boost your immune system

The importance of an efficient immune system in the body has led researchers to study the different methods of improving or boosting this system. Due to the immune system being a complex mechanism of various cells and organs, a specific drug or diet that can single-handedly boost immunity is yet to be discovered. However, diet, exercise, and an overall healthy lifestyle can help keep this system well balanced and functional.


Being one of the main sources of the entrance of microbes, the gastrointestinal tract makes up an important part of the immune system and resides approximately 80% of the immune cells and most of the normal microbiota. Since the gut makes up one of the first defenses against pathogens, a healthy gut microbiota, which is dependent on the diet, is necessary for the normal functioning of the immune system. The following foods help boost the normal functioning of the immune system along with a healthy well-balanced diet.


The significant effects of physical exertion on immunity have been known and studied for decades and have been associated with a decreased risk of cardiovascular and respiratory conditions. This association between the central nervous system, endocrine system, and the immune system is also seen in the release of different hormones like catecholamines, somatotropin, and cortisol, etc during exercise. More specifically the number of neutrophils and lymphocytes was seen to increase significantly during exercise with a fall in CD4: CD8 ratio. These beneficial changes can be obtained by incorporating both simple exercises like walking and yoga and high-intensity work-outs like cycling.

Lifestyle Choices

Another way to help the immune system along with the dietary changes is to adopt a healthy lifestyle. This includes abstaining from smoking as cigarette smoking significantly increases the risk of numerous diseases and harms both the innate and adaptive immune systems. A recent study showed a decrease in CD4 T-cells, regulatory T-cell, dendritic cells as well as different immunoglobulins in smokers and may also increase the risk of autoimmune conditions.

Smoking was also seen to enhance synthesis of inflammatory mediators and cytokines that promote inflammation and can lead to poor healing. A decrease in alcohol intake is also advised as too much alcohol can lead to harmful effects on the immune system and can further lead to conditions like pneumonia, ARDS, liver disease, and even some cancers.

Alcohol harms the immune system in multiple ways with a resultant decreased ability of the defense system to prevent, fight, and recover from infections. At its entry point, alcohol destroys the natural microbiota of the gut which is fundamental for the normal development of the immune system. It also affects the action of cilia in the respiratory system and adversely affects the alveolar immune cells which allow pathogens to easily enter the lungs.

An adequate amount of sleep is also necessary as the production of T-cells and various cytokines is increased during sleep and it also helps in forming the immunological memory. Sleep is also a good way to relieve stress and chronic stress is an important factor for immune dysregulation. Chronic stressors are seen to cause more severe and recurrent infections due to their deleterious effect on the immune response. Adopting preventive measures such as hand hygiene and staying up to date on your vaccination is also helpful in avoiding infections.