A Brief Introduction To Immune System.

We are constantly bombarded by the army of bacteria, fungi and viruses, yet most people do manage to stay healthy most of the time. This is because almost every part of our body is protected by a complex immune system, that is our defence system which protects us from invading organisms, toxic by-products of our metabolic processes, such as free radicals, tissue damage, growth of cancerous cells, etc. All these disease producing agents or micro-organisms are called pathogens. Inflammation is a consequence of any immune response – it is necessary in order to quickly increase the amount of defencive cells. However, this process should be controlled, otherwise the immune system can turn on itself and become a cause of chronic inflammation and auto-immune diseases.

The immune system has a different ways of monitoring the contact with foreign bodies through natural barriers, such as skin and mucous membranes, that contain lymphatic clusters that act as checkpoints. In the gut (GALT – Gut Associated Lymphoid Tissue) these clusters are called Peyer’s patches, that are also found in the throat, tonsils and adenoids. The appendix is also part of this system. Another defence system is in the skin (SALT – Skin Associated Lymphatic Tissue).

Our immune system is intelligent and can adapt to various conditions. It can recognise, learn, communicate, repair and attack. The cells responsible for the various immune defence strategies are leukocytes (white blood cells) and their derivatives. They circulate in the blood stream and have the ability to migrate into tissues when needed. Leukocytes are also found in lymphatic system. Almost all white blood cells originate from stem cells in bone marrow. The main types of these cells are Monocytes, Neutrophils, Eosinophils, Basophils and Lymphocytes.

  • Monocytes are “garbage collectors”. They can migrate into tissues where they transform into macrophages and are responsible for uptake of microbes and particles followed by digestion and destruction of this material. This process is called phagocytosis. Macrophages are concentrated in the tonsils, adenoids, Peyer’s patches, spleen,  bone marrow, lungs and lymph nodes, as well as roaming free in the blood. Macrophages destroy invaders by dissolving their membrane using enzymes. They can also produce superoxides, nitric oxide and hydrogen peroxide if necessary. Macrophages also transmit the information about the invader back to lymphatic system, interacting with and activating adaptive immune system. Monocytes manifest themselves in fever.
  • Neutrophils are also phagocytic specialists and mini-macrophages. They are less powerful, but greater in quantity. They convert oxygen to superoxide radicals and secrete a protein (lactoferrin) which binds with iron, making it unavailable for bacteria. Neutrophils remove waste matter, such as debris, viruses, bacteria, fungi and other pathogens from the body and manifest themselves in further inflammation and fever.
  • Eosinophils produce chemicals that destroy parasites and are involved in allergic reactions. They secrete an enzyme that breaks down histamine. An increase of eosinophils manifest itself in hay fever and allergic asthma.
  • Basophils release histamine and heparin and also involved in allergic reactions. They are also present when damaged skin is healing.
  • Lymphocytes can be Natural Killer Cells, B-cells or T cells. T cells mature in the thymus gland and are responsible for cell mediated immunity (immune response that does not involve antibodies). B cells mature in the lymphoid tissues and the wall of the intestines and are responsible for the production of antibodies. Natural Killer Cells can kill a variety of infectious microbes, cells that have become cancerous or infected with viruses. They are found in spleen, lymph nodes, red bone marrow and blood.

There are two types of immune system: innate (non-specific) and acquired (specific). We are born with innate immune system and we develop acquired immunity to certain diseases and viruses as we get exposed to them during our lifetime. Innate immune system is our first line of defence and is present in the barrier structures, such as skin, gut and respiratory membranes. The secretions of both skin and mucus membranes are mildly antiseptic, while other defensive agents include “friendly” bacteria that normally lives on our skin and in mucus membranes. They live at peace with our body and keep hostile bacteria and fungi at bay. If these bacteria are removed by antibiotics or a disease, the germs and parasites that are not normally dangerous and dormant, become the cause of debilitating disease and may even be lethal. This is why one should think twice before consider an antibiotic course. Overall, innate immune system is responsible for identifying what is good or bad, and once invader or injury is detected it provides a non-specific response to isolate the intruder and to reduce its damage, while waiting for the slower, but more targeted, specific immune system to eliminate the problem.

Adaptive immunity provides a selective (targeted) attack, that normally triggered by a memory of prior exposure to a certain pathogen. This acquired immune system evolves as we grow and come into contact with foreign invaders. It consists of B and T lymphocytes. B cells release large quantity of antibodies (plasma proteins) known as Immunoglobulins. There are many types of B cells and all are capable of releasing specific antibodies that attach to specific pathogens. Those who ever looked at their blood test results might often find things like IgA (slgA), IgM, IgG and IgE. IgA (slgA) is a specialised group of antibodies that exist mainly in the intestines, but also in respiratory and genitourinary mucus membranes. It accounts for 70% of antibodies produced each day. IgM and IgG are circulating antibodies, that produce specific responses against some viruses and bacteria. IgE is involved in the allergic response and causes certain cells to release granule like structures rich in histamine and heparin. Any substance that can provoke antibody activity is called an antigen.

T lymphocytes are also activated in response to specific antigens. They regulate the rest of the immune system, and some of them – Cytotoxic T Cells – play a role in destroying pathogens by binding to enemy cells, secreting special protein called perforin to puncture the cell wall and pouring in special granules (granzymes) to destroy it. Quite clever isn’t it? However, sometimes it goes out of hand and causes trouble. For example there are helper T cells: TH1 ans TH2 – they can be seen in certain blood test results too. TH1 promotes cytotoxic activity to help destroy invaders, but if predominant, it can cause chronic inflammation or even auto-immune disorders. TH2 promotes antibody production, but if predominant may result in allergies. Cytokines are important molecules involved in regulating the immune system. I will write about what causes inflammation and immune dysfunction in later posts.

The two types if immune system described above are linked to two types of immunity in general: active and passive. Active immunity is gained by previous exposure to the infection. An antibody produced to fight the infection, is then remains in the body ready to fight the same organism again. Vaccinations are also building up active immunity. Passive immunity is acquired from mother to baby, when antibodies being passed to the baby during development in the womb and during the first days of breastfeeding. This is why breastfeeding is important.

Immune system can also be cellular and humoral. Cellular one is mediated by active cells such as white blood cells, macrophages and mast cells. The humoral immunity is mediated by protective substances circulating in the blood or tissue fluid. Humoral agents include two natural antibiotics: lysosome and interferon. Lysosomes attack the walls of many bacteria and kills them and interferon inhibits reproduction of viruses in infected cells. Its inhibiting effect can then be applied anywhere else in the body. Once its active, a person becomes temporary immune to attack by other similar viruses, e. g. one is unlikely to develop measles during an attack of chickenpox.

Chronic stress, poor diet, being overweight or underweight, malnutrition, smoking, drinking, sugar, pollution, depression, lack of sleep, infections, etc. can prevent immune system from working properly. In later posts, I will write in more details about what goes wrong and what can be done to prevent it.

 

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