The human body is an amazing machine that has evolved into a multicellular and complex structure from as single cell. It is self-sufficient and performs every physical, biological, chemical function without any glitch. The human body takes the required vitamins and minerals to carry out the basic body functions and mechanisms from healthy food and drinks that we consume. The healthy body is like Fort Knox, with an excellent defense mechanism to overcome any external threat by pathogens or toxic substances. This defense mechanism is called immune system. An active immune system prevents bacteria, viruses, fungi as well as other harmful pathogens from entering our body as well as fighting the ones that have entered. The immune system consists of various cells, tissues and organs that work together to keep the body protected from pathogens. Let us have a look at the various organs and cells of the immune system. Organs of the Immune System The major tissues and organs involved in the immune processes are mainly classified into primary and secondary lymphoid organs. The primary lymphoid organs include bone marrow and thymus, whereas the secondary lymphoid organs include the lymph nodes and spleen. Bone Marrow The bone marrow is the production house for blood cells, namely B cells, natural killer cells, immature thymocytes, red blood cells and platelets. This process of production of blood cells is called hematopoiesis. Thymus This is the site where immature thymocytes migrate and develop into mature T cells. In this bilobed organ located above the heart occurs a process of selective deletion of T cells that show immune activity against the body itself. Thus, the T cells released out from the thymus are well-equipped to attack the foreign bodies and\or pathogens only, and not the cells or tissues of the body itself. Lymph Nodes These encapsulated bean-shaped structures are the site for filtration of lymph and trap antigens that enter the tissue spaces. The antigens are filtered through the lymph nodes and the macrophages and dendritic cells capture these antigens and present them to the T and B cells, thus evoking a full-fledged immune response. Spleen Located in the abdominal cavity, this oval-shaped organ specializes in filtering out the antigens present in blood. This is also a site where the interaction between the antigens and the immune cells occurs. It has zones of tissues filled with B cells producing antibodies against the trapped antigens, macrophages and dendritic cells that engulf the antigens and present a part of them on their surface. It is also serves as the destruction site and graveyard for old red blood cells. Cells of the Immune System Lymphocytes are the major cells of the immune system, and are responsible for diversity, specificity and memory creation. They are aided by and triggered by several other white blood cells. B Cells These cells develop and mature in the bone marrow and are responsible for the synthesis of proteins called antibodies. They are defined as 'specialized proteins that specifically recognize and bind to a particular antigen'. This binding of the foreign body signals other cells to kill or engulf the unwanted substance. They are displayed by B cells on the cell surface and are also released into blood. T cells These cells are produced in the bone marrow but develop and mature in the thymus. On the basis of specialized molecules present on their surface, T cells are mainly classified into T helper cells (Th cells) or cytotoxic T cells (CTL). Th cells have molecules called CD4 (cluster of differentiation 4), whereas CTLs have molecules called CD8 (cluster of differentiation 8). The T helper cells are used to potentiate immune responses by secreting specialized factors that activate the white blood cells to ward off a possible infection. The CTLs are directly involved in killing the tumor cells and infected cells. Macrophages These are cells capable of engulfing foreign particles, viruses, bacteria, parasites etc., killing them and presenting derivative antigens on the cell surface. Such cells are termed professional antigen presenting cells (APCs). They not only kill the pathogen but also educate and activate other cells to mount a response against it. Dendritic Cells These also serve as professional APCs and are more efficient than macrophages as far as antigen presentation is considered. They capture the antigens and bring it to lymphoid organs initiating an immune response. In addition, they are also involved in selecting the self-reactive immune cells, and inducing suicidal mechanisms in such cells. Natural Killer Cells These killer cells are true to their name as they kill tumor cells, virus-infected cells like herpes and cytomegalovirus infected cells, as well as parasite-infected cells. These cells can identify antibody-bound foreign antigens and pathogens, and thus selectively engulf and destroy them. Granulocytes These are a set of white blood cells characterized by the presence of granules in their cytoplasm and lobular nuclei. This granular cytoplasm gets stained differently with different dyes, which forms the basis for their classification into eosinophils (get stained with acidic dye), basophils (get stained with basic dye) and neutrophils (both acidic and basic dyes). Neutrophils and eosinophils engulf foreign substances and destroy them using destructive enzymes present intracellularly. Basophils are nonphagocytic cells and are responsible for releasing active substances that give rise to an allergic response. Interesting Facts about the Immune System

• The immune system has non-specific and specific barriers to deal with foreign particles and pathogens. The non-specific barriers are collectively termed 'innate immune system' and encompass anatomical, physiological as well as cellular barriers. The skin and mucous membranes serve as the mechanical barrier for entry of pathogens. The physiological barriers refer to the body temperature, pH as well as enzymes and specialized non-specific proteins. Cellular non-specific response includes phagocytosis and inflammatory reactions.
• The specific immune reactions that occur in the lymph nodes, blood and other body tissues are termed 'adaptive immune system', and is required when the innate system fails to prevent the entry or growth of the foreign body. However, in general most of the microbes are readily cleared by the innate immune system.
• At times, the immune system goes into a 'hyperdrive' and reacts inappropriately against certain antigens. This is termed 'hypersensitivity', and leads to an allergic reaction characterized by reddening and rashes at the site of contact as well as sneezing.
• There are four phases of an immune response: recognition, activation, amplification of defense, suppression, and memory creation.
• The environment plays an important role in affecting our immune response. Toxic substances, air pollution, pesticides and cigarette smoke affects the defense system of our body.
• The circadian rhythms and sleep-wake pattern influences immune function. Lack of sleep has been shown to alter leukocyte numbers, proliferation and function.
• Each microliter of blood has about 4500 to 10,00 white blood cells, that is, over 50 billion cells in the entire body.
• One may lose about five million WBCs when donating blood, and still be left with enough to fight a full-fledged immune response.
• Stress affects the normal immune functions including reduction in the activity of NK cells and lymphocytes as well as antibody production. This occurs through stress hormones like cortisol, norepinephrine etc. Such suppression of the immune system makes the body more susceptible to infections and may also lead to reactivation of latent viruses.
• Hence stress-busters like massages, relaxation techniques etc. not just help to relieve psychological stress, but also help to maintain the efficiency of the immune system.
• Similarly, an unhealthy and/or poor diet as well as crash diets also lead to weakening of the immune system.
• Pumping excessive iron in the gym makes the body produce more cortisol and adrenaline. These hormones temporarily impair the immune function.
• Too much of anything is always harmful. Similarly, some sunlight helps the body produce vitamin D and too much sunlight, UV radiation to be precise, suppresses the immune system.
• After an insect bite or mosquito bite one observes a red, itchy bump on their skin. This is a sign that the immune system is at work, and acting against the foreign substances that have entered through saliva of the insect.
• 'Laughter is the best medicine.' This is not just an old saying but laughter induces a proactive immune response that leads to a healthy body.

Our body has a defense mechanism that is stronger, more organized and more efficient than the strongest armies in the world. In order to keep the body's defense system impregnable, it is essential to follow a nutritious diet and exercise routine as well as avoid addiction to tobacco, alcohol and narcotics. Stay stress-free and keep laughing for a stronger immune system.