The human immune system has evolved into a complex defense mechanism that serves to fight off external insults, pathogens, and other immune stressors in the most efficient way possible. When everything is working correctly, the interplay of both physical and chemical hurdles that the immune system puts up as its various lines of defense effectively neutralize pathogens before they are able to cause severe disease. The goal is to isolate the offending agent and swiftly remove it from the body. Here, we will look at the various components of the immune system and how they function to provide comprehensive protection.

Immune System Main Organs

Several organs in the body facilitate the immune system’s response to infection. The key immune system organs include the thymus, the spleen, the lymphatic system and lymph nodes, and bone marrow.

Thymus Gland

The thymus is a lymphoid organ located behind the sternum in the chest. The main thymus function is as a site of maturation for T-cells of the immune system, which are primarily involved in the adaptive immune response. It is most active from before birth to the period of adolescence in humans, when it is cranking out mature T-cells for the body. During adolescence, the organ begins to shrink in size. The majority of thymus derived T-cells are therefore produced early on in life although the organ retains some functionality through adulthood.

Spleen

The most well-known role of the spleen is to remove and recycle damaged or old red blood cells and recycle iron. However, it is also a critical organ for the function of the immune system as a key part of the body’s lymphatic system. It acts as the main filter for blood-borne pathogens and antigens, helps to remove cellular waste, and facilitates the manufacture and activate several white blood cells of the immune system. As a site of antibody production, it removes antibody-coated bacteria and blood cells through the lymph nodes, allowing the generation of immune responses that protect the body against disease-causing bacteria, viruses and fungal infections.

Lymph Nodes

Lymph nodes are located throughout the body and are lumps of tissue that are rich in white blood cells. As essential components of the immune system, they filter lymph fluid which contains waste products from bodily tissues. In doing so, they help trap viruses, bacteria, and other organisms to help fight infections. Lymph nodes contain immune cells known as lymphocytes, which are key cells involved in the immune response to bacteria and other pathogens.

Swollen Lymph Nodes

When lymph nodes are swollen, they can be indicators of present viral and bacterial infections, while in rare cases can also indicate more severe conditions, including cancer. If swollen nodes are persistent, it’s wise to discuss this with your healthcare provider to rule out serious health issues.

Bone Marrow

The bone marrow is a critical component of the immune system as all immune cells originate in the bone marrow. B lymphocytes of the immune system remain in the bone marrow to mature while T cells originating there travel to the thymus.

Now that we have an understanding of the key organ components involved in immune system function, it’s important to understand how the immune response works. In general, immune system function is divided into two arms, the innate immune response and the adaptive immune response, but there are other critical features of the immune system that are worth highlighting.

Physical Immune Barriers

The body contains several physical barriers that function as the first line of defense against pathogen invasion. These include our skin as well as the mucosal linings, specifically including the mucosa lining the digestive tract and the lining of the respiratory tract and lungs. For a pathogen to infect, it needs to penetrate these physical immune barriers. As long as these barriers are intact and healthy, they provide an effective defense against penetrating organisms that the body encounters on a daily basis. In addition, these physical barriers are protected further by the presence of antimicrobial proteins and chemicals. One of the most effective of these chemicals is stomach acid itself, which kills numerous organisms before they have a chance to penetrate the digestive tract.

The Innate Immune Response

If a pathogen survives the physical and antimicrobial defenses it has encountered to this point and is able to pierce the lining of the intestines and cross the epithelial barrier to begin to replicate, the innate immune response kicks in. This is the immune system’s non-specific response with the goal being to do whatever is necessary to neutralize the pathogen. The innate immune response involves immune cells including neutrophils and macrophages, which become activated to ingest and destroy the pathogen. The most important antimicrobial component of the innate immune system is known as the complement system. This is comprised of more than 30 molecules that are activated at the first sight of the pathogen. These molecules lead to direct killing by physically punching holes in bacterial membranes, generating a localized inflammatory response, and through enhancing phagocytosis, the process of engulfing pathogenic organisms. In addition, a network of cytokines and chemokines, chemical messengers of the immune system initiate a process known as chemotaxis by attracting more cells of the innate immune system to the focal area of the infection as reinforcements.

What is Phagocytosis?

The primary goal of the innate immune response is to deliver phagocytic cells to the source of the infection. As phagocytosis is the process of engulfing pathogens to neutralize them, neutrophils are the first responders and initiate a response within minutes of detection. They release several reactive oxygen species – also known as free radicals – that have antibacterial properties. The next class of phagocytic cells that are activated are the macrophages. Both neutrophils and macrophages engulf and internalize pathogens to ultimately kill them and remove them from the body.

The Adaptive Immune Response

The innate immune response is effective at clearing most immune insults before serious disease can occur. However, in the cases where it’s unable to clear the infection, cytokines that are released trigger the activation of the adaptive immune response. While the innate response is generalized, the adaptive immune response is specific and targeted to the offending pathogen. It involves the transport of antigens, or proteins that are specific to the organism, to lymph nodes for presentation to T cells of the immune system. This generates one of two responses by the adaptive arm of the immune system: the production of antibodies specific to the antigen, or a cell-based (cytotoxic) response. Whether one response is generated or a combination of both is initiated, the immune system targets the offending pathogenic organism to clear it from the body.

Another important feature of the adaptive immune response is the concept of immunological memory. A small number of the antigen-specific T cells and B cells which are generated during the adaptive response develop into memory cells. The logic behind memory cells is so that the immune response to a second or subsequent infection by the same organism is quicker and highly targeted. Since the immune system now recognizes the offending pathogen, the response to it is nimbler and the immune system is primed and ready to deal with the immune insult effectively.

The immune response to an offending agent is complex and the immune system has evolved into a fine-tuned machine that effectively serves to protect the health of the host and maintain vigilance. By doing so, it prevents harm and reduces the risk of adverse outcomes to the body. Keeping the immune system healthy and functioning at peak levels is, therefore, of primary importance. A healthy diet and lifestyle along with smart supplementation can go a long way towards keeping the immune system in tip top shape.

Reference

Kellie S and Al-Mansour Z. (2017) Overview of the Immune System. Nanotechnology in Vaccine Development, 63–81.