IMMUNE SYSTEM

It is an immediate response against different pathogens and include first- and second-line defenses. Intact skin and mucosal membranes represent the first line barriers. Immune cells and proteins are the second line which suppress pathogens from spreading all over the body. The most important reaction of it is inflammation. 

Innate immune system is an immune response to different types of invaders in the same way, that is why it is called unspecific. This response happens immediately or after few hours of infection. Innate immune system consists of first line defenses which are barriers prevent pathogens from entering the body. So, we were born with these defenses like skin and mucous membranes. Other defenses are specific proteins produced by body cells. These innate mechanisms reduce the workload of the adaptive defense mechanisms.

Almost all body systems have first line defense represented as barrier against external harmful substances. Skin covers the majority of body and extends over a large area. Respiratory and gastrointestinal tracts have inner lining with epithelial tissue called mucosal membranes. They protect internal cavities from external pathogens. Respiratory tract especially bronchi lined with cilia in addition to mucosal membranes; that helps in eliminating wastes from the respiratory system. 

Some processes are involved in getting rid of unwanted wastes, such as diarrhea and vomiting from gastrointestinal system, and coughing and sneezing from respiratory system. 

Actually, not all germs are simply harmful, instead human body contains normally helpful germs called flora. Flora is most prevalent in Nasopharynx, Mouth and throat, Skin, GI tract and Vagina. They function by competing other pathogens on cell receptor sites. And they contribute to producing vitamins such as Vitamin K and B12.

Physical barriers described above contain also chemicals prevent pathogens from prefiltration; these include Ph level of skin between 3 and 5, sebum has antimicrobial activity, lysosomes (containing digestive enzymes) found in saliva lacrimal fluid, mucus secreted by specialized cells in the digestive and respiratory systems prevents entering of microorganisms, and finally, the very low Ph level of stomach produced by HCl prevents the digestive system from swallowed pathogens with food. 

Along the respiratory system there is ciliated lining acts as preventive method from various substance human breathing. Nose contains mucus-coated hairs help in trap inhaled particles. Cilia presented in lower respiratory passages act as a cleaner for contaminated mucus in the respiratory system. 

Along the respiratory system there is ciliated lining acts as preventive method from various substance human breathing. Nose contains mucus-coated hairs help in trap inhaled particles. Cilia presented in lower respiratory passages act as a cleaner for contaminated mucus in the respiratory system. 

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In addition to previous defenses, the body use also cellular and chemical methods represented by the inflammation response, which consists of 4 elements: redness, heat, pain and swelling, and various proteins in body fluids, and certain cells like Phagocytes and natural killer (NK) cells.

In humans, phagocytosis is chiefly a defensive mechanism against different antigens. Many immune cells contribute in the process of phagocytosis, macrophages basically are cells present in different organs like liver, spleen and lymph nodes, and they are dominant in chronic inflammation. Neutrophils are circulated cells in blood. When they encounter infected area, they become phagocytic, that is why it is mostly activated during acute inflammation. Eosinophils are mostly found during parasitic infections like warms. B-lymphocytes are transformed to plasma cells after phagocytosis. Finally, dendritic cells are responsible for exaggeration of some immune responses. The mechanism of phagocytosis consists of engulfing foreign antigen, then the antigen is totally surrounded by a membrane-bound vacuole called a phagosome, after that it is digested by the immune cell. 

The cells of natural killer cells are type of innate lymphocytes; it is considered as granular lymphocytes (LGL). They are first observed as cancer killer. It is now known that NK attacks malignant cells and virus-infected cells. When it is bound to antigens, NK cell secretes certain chemicals and perforins lead to kill the antigen. It has also role in regulating immune response by activating or inhibiting immune processes. 

Hypothalamus represents body thermostat. When an invading microorganism enter the body, hypothalamus rises the baseline temperature, which leads to fever. Substances that trigger fever is called pyrogens, they could be bacteria or toxins. Macrophages and other immune chemicals also provoke fever. As an immune response, fever could be helpful by triggering the liver and spleen to prevent iron and zinc needed by microorganisms, and by increasing metabolic rate, thus, speeding recovery. On the other hand, high fevers could be dangerous by causing brain damage or even death. 

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After being exposed to innate immune system, foreign antigens will face adaptive immune system if they have escaped. Each foreign substance would face specified response by adaptive immune system. That is why it is considered as antigen-specific; it is also capable to rememorize previous immune responses, and it has systematic effects. Because of its memory, the adaptive or acquired immune system, acts with maximal effects when a known antigen is exposed to it. This principle applies for vaccinations. It gives highly specific and faster response. Adaptive immune system has two subtypes, Humoral, or antibody-mediated immunity and Cellular, or cell-mediated immunity.

B-lymphocytes express antibodies on their surface. When an antigen is exposed to an appropriate binding site, it would be linked to the B-cell. Multiple antigens could be linked to an antibody, but each B-cell express a unique type of antibodies. Antibodies produced by plasma cells are called immunoglobulins. Immunoglobulins circulate in blood to detect any microorganisms or toxins. That represents humoral immunity role in antigen detecting and processing. After an antigen is linked to an antibody, a cascade of immune responses is activated, which involves opsonizing the pathogens, facilitating complement function, and exacerbating production of specific antibodies. Helper T lymphocytes have also indirect role in humoral immunity, they promote phagocytosis by inducing macrophage activity, and amplifying B and T lymphocytes production. 

In this type, cells attack antigens directly, i.e., without the need of antibodies. It is mostly activated in cases of virus, intracellular bacteria, fungi and protozoans’ infections, in addition to malignant cells. Cell involved in cellular immunity are phagocytes, antigen-specific cytotoxic T-lymphocytes, and the release of various cytokines in response to antigen. 

They are foreign substances induce immune response in the body. They considered as foreign molecules (nonself), and they are usually large and complex, and being attacked from the immune system. 

Antigen-presenting cells: play a supporting role in immunity response. It presents antigens for recognition by T-cells. It is not specific against certain types of antigens. 

T and B lymphocytes: They form the cell-mediated arm of immunity. T lymphocytes contribute to cellular immunity (non-antibody) and B lymphocytes contribute in humoral immunity

Although B and T lymphocytes have different sites for maturation, but they are both produced in bone marrow by the same progenitor cell. Thymus is responsible for maturing T-cells, while bone marrow is responsible for maturing B-cells. When the progenitor cell reaches the appropriate cite for maturation it becomes immunocompetent. 

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They represent a group of conditions in which some of the immune system components can not react or function properly against invading pathogens. It is an insufficient response of the immune system opposite to allergic condition in which an over reacting response is happened. Immunodeficiencies could be congenital or primary as the inherited conditions that could be further classified depending on the abnormal component, or acquired, secondary due to extrinsic factors. 

They are secondary conditions lead to depressing functions of the immune system. AIDS is a common example which caused by HIV. HIV interferes helper T (CD4) cells activity. This causes opportunistic infections, i.e. infections exploit the weak immune system to be activated. Some examples include pneumocystis pneumonia and Kaposi’s sarcoma. Hodgkin’s lymphoma also leads to immunodeficiency by affecting lymph nodes. 

AIDS is an abbreviation for acquired immunodeficiency syndrome, which is a chronic condition caused by human immunodeficiency virus (HIV). AIDS progresses through 3 stages. HIV interferes the immune system function by attacking T-helper cells and depresses cell-mediated immunity. It multiplies in lymph nodes during regression period and then appear as symptomatic disease after months to 10 years. There is no cure for AIDS, but treatment help in slowing and stopping advance of disease. Medications include reverse transcriptase inhibitors (AZT), protease inhibitors (saquinavir and ritonavir). Other classes are currently developed that could work on blocking HIV’s entry to helper T cells. Preventive measures are of great importance which include avoid sharing needles or syringes and avoid unsafe sexual relationships because HIV is transmitted via body fluids – blood, semen, and vaginal secretions.

Normally, the immune system differentiates between self and harmful antigens via complex processes. In some instances, the immune system fails to recognize self-tissues, leading to attack them as foreign bodies by producing autoantibodies. This leads to the presence of Autoimmune diseases. Examples include multiple sclerosis, myasthenia gravis, Graves’ disease, Type I diabetes mellitus, systemic lupus erythematosus (SLE), glomerulonephritis, and rheumatoid arthritis.

It is an inappropriate response of immune system against certain antigens called allergens. Hypersensitivity includes many types of allergic reactions. It usually happens due to disruptions in immune regulation system. It is not obvious what are the direct causes of hypersensitivity but there is evidence suggests the role of genes. Hypersensitivity could be activated by endogenous and exogenous antigens. These antigens are handled as invading harmful pathogens.  

An allergy reaction happens when the body responds to some substances as foreign antigens, that could be not harmful to other people. It is considered as an exaggerated response from the immune system.    

Some individuals may respond to certain substances like food (as peanuts, eggs or crustaceans) or venom (bee sting), or even medications (sulfa drugs) by severe allergic reaction called Anaphylaxis or anaphylactic shock. It happens suddenly in 20 minutes of exposure, and it is mediated by chemicals like antihistamines. Anaphylaxis requires immediate management via epinephrine, or it could be life-threatening.  

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  a. Innate Immunity 

  b. Acquired immunity 

  c. Passive immunity 

  d. Active Immunity

  a. Innate Immunity 

  b. Acquired immunity 

  c. Passive immunity 

  d. Active Immunity

  a. B-cells 

  b. T-cells

  c. Both B and T cells

  d. Mast cells

  a. Physical barrier 

  b. Physiological barriers

  c. Cellular barriers 

  d. Cytokine barriers 

  a. Autoimmune diseases

  b. Allergy

  c. Immunodeficiencies

  d. Hypersensitivity