Hello everyone , I love explaining about my knowledge in Biology because i am medical student :) & this is my first post related to science.Hope you will have a feel that you learned something today :) . so lets get started , #### Introduction :- Every day we are exposed to a large number of infection agents . However only a few of them results in diseases .*why* ? It is due to the fact that the body is able to defend itself from most of them . The overall ability of an individual to fight against the disease causing organisms is called *Immunity*. ![10-Amazing-Facts-About-Your-Immune-System-722x406.jpg](https://steemitimages.com/DQmRdynDhULEiRHbW4aSVpvjWBRmNyGYgSSKJeGHf53uCd5/10-Amazing-Facts-About-Your-Immune-System-722x406.jpg) [Source](https://www.tradingview.com/chart/STEEMUSD/3kPrLof9-HODL-Steem-chart-going-Long/) * The network of organ cells & proteins that protect the body from harmful infection agents such as *Bacteria,Viruses,Animal parasites,Fungi* etc., is called the *Immune system*. * The basic requirement of the immune system is to differentiate between self and non self & to protect the body from [*Harmful foreign substances*](https://medlineplus.gov/ency/article/000821.htm),[*Micro-organisms*](https://en.wikipedia.org/wiki/Microorganism),[*Toxins*](https://en.wikipedia.org/wiki/Toxin) & [*Malignant cells*](https://www.cancer.gov/publications/dictionaries/cancer-terms?cdrid=45771) etc., ##### Concepts of Immunology :- Study of Immune system is called [*Immunology*](https://en.wikipedia.org/wiki/Immunology) To understand Immunology better,we have to know certain basic concepts such as lines of defence,Cells,Organs & Soluble mediators of the immune system , antigens , types of immunity , mechanism of immunity & Immunological Disorders . ##### lines of defence in body :- ![ed22e4862373dc32714f14be7a701cc4.jpg](https://steemitimages.com/DQmaHWL6fdsUb5dFhQMZtXJFJZeeAn3FHeTgEfP9En37CWN/ed22e4862373dc32714f14be7a701cc4.jpg) [Source](https://i.pinimg.com/originals/ed/22/e4/ed22e4862373dc32714f14be7a701cc4.jpg) * ###### First line of defence : Whenever bacteria , viruses , Fungi and parasites try to enter the body of an organism , skin , mucous membranes & the Enzyme Lysozyme of saliva , tears etc., present their body . * ###### Second line of defence :- If the microorganisms cross 1st line of defence & enter the body ,the phagocytes,Natural killer cells,Antimicrobial substances, inflammation, fever etc., * ###### Third line of defence :- If the microbes cross even the 2nd line of defence , then the lymphocytes & antibodies fight against them . It is highly specific but takes several days to become fully functional. *If all the Three lines of Defence fail, it results in diseases*. ##### Cells of the immune system :- ![58eef5e8c912c6bd41b9e603cf4df210.jpg](https://steemitimages.com/DQmXM79r7q4fKgJH3g8NrwoXBEif66kTEvAsPhg7pSna2Gx/58eef5e8c912c6bd41b9e603cf4df210.jpg) [Source](https://i.pinimg.com/originals/58/ee/f5/58eef5e8c912c6bd41b9e603cf4df210.jpg) There are three types :- 1) ###### Lymphocytes :- These are the chief cells of the immune system ,which are of 3 types * B-cells * T-cells * Larger Granular Lymphocytes (LGLs) Based on the size , [lymphocytes](https://www.healthline.com/health/lymphocytes) can also be divided into small & large Lymphocytes .Small Lymphocytes include B-cells & T-cells whereas the large lymphocytes include granular lymphocytes that consists of *Natural killer cells*(Nk-cells). ###### B-cells :- * The lymphocytes capable of producing antibodies and can capture circulating antigens are called [B-cells](https://en.wikipedia.org/wiki/B_cell). * They are produced by the "[*Stem cells*](https://en.wikipedia.org/wiki/Stem_cell)" in the bone marrows of *Adult mammals & [Bursa of Fabricius](https://en.wikipedia.org/wiki/Bursa_of_Fabricius) in Birds*. ![369897.image0.jpg](https://steemitimages.com/DQmZBXCpwbfQx9wwPv5muvd1nW8ZYNmP5CATTdMFsccEdyu/369897.image0.jpg) [Source](http://d2r5da613aq50s.cloudfront.net/wp-content/uploads/369897.image0.jpg) Mature B-cells synthesize various types of antibodies which are displayed on their Membrane surface . Also called as Immuno-competent B-cells.These mature Immuno-competent B-cells reach the secondary Lymphoid organs & develop into Functional Immune cells which later differentiate into "*Long Lived* " memory cells & Effector plasma cells .[Plasma cells](https://en.wikipedia.org/wiki/Plasma_cell) produce antibodies specific to the antigen to which they are exposed .Memory cells store information about the specific antigens & Show quick response when the same type of antigen invades the body again . ###### T-cells :- The lymphocytes that are not capable of producing antibodies & cannot recognize the free/ circulating antigens are called T-cells . * They are produced in the bone marrow reach the Thymus gland & differentiate into mature [T-cells](https://en.wikipedia.org/wiki/T_cell) . ![250px-Illu_thymus.jpg](https://steemitimages.com/DQmQvFgH9dXAkAdLTAqRfY9SGNyFBnsTJppwV4oC6dP48KM/250px-Illu_thymus.jpg) [Source](https://upload.wikimedia.org/wikipedia/commons/thumb/c/cf/Illu_thymus.jpg/250px-Illu_thymus.jpg) ###### Do you know ? Through T-cells cannot recognize the free or circulating antigens , they can recognize antigens if presented by *[Antigen presenting cells](https://en.wikipedia.org/wiki/Antigen-presenting_cell)(Apcs)* or altered self cells by MHC molecules.APCs include dendritic cells , macrophages & B-lymphocytes . ##### Soluble mediators of Immunity :- There are three types namely : * ###### Complement proteins ![complement-system.jpeg](https://steemitimages.com/DQmZ8vnSK2BoAwK8Vt9wQpbpPNYUi8xZq512TR5u6ZeRB3T/complement-system.jpeg) [Source](http://images.tutorvista.com/content/immune-system/complement-system.jpeg) These are a group of Inactive Plasma proteins & cell surface proteins.When activated they form a [Membrane attack complex](https://en.wikipedia.org/wiki/Complement_membrane_attack_complex) (MAC) that forms pores in the plasma membrane of the affected cells allowing ECF to enter the cells & make them Sell & Burst . Some of them form a coat on the surface of the pathogens & attract Neutrophils & Macrophages to phagocytes & them destroy them. * ###### Cytokines ![Closer-Look-Cytokines-Blog-Sidebar-Final.jpg](https://steemitimages.com/DQmPhELZTmHLpE3kAgfc53e3eee6fTrxTNLoVyXYSSsMFum/Closer-Look-Cytokines-Blog-Sidebar-Final.jpg) [Source](https://data.integrativepro.com/images/blog/Closer-Look-Cytokines-Blog-Sidebar-Final.jpg) They are small,soluble Molecules secreted mostly by the Th cells or infected cells .They bid to cell surface receptors & initiate activation or differentiation of the cell of the immune system to stimulate Phagocytosis & cyclosis of the infected cells . ###### Antibodies ( Immunoglobins ):- Whenever *[Pathogens](https://en.wikipedia.org/wiki/Pathogen)* enter our body , the B-cells produce an army of proteins called antibodies to fight with them . * They are Highly specialized for binding with specific antigens. * The part of an antibody that recognizes an antigen is called [Paratope](https://en.wikipedia.org/wiki/Paratope)(Antigen binding site). Based on this mobility,antibodies are of two types : * Circulating (or)Free antibody * Surface Antibody The circulating are present in the body fluids whereas the surface antibodies are present on the surface of the mature B-cells as well as memory cells . ##### Structure of Antibody ![antibody.gif](https://steemitimages.com/DQmQyw3oNYQ965ZW3p3WEweunri4cxTxsTGRX72YpnoCNBd/antibody.gif) [Source](http://www.biology.arizona.edu/immunology/tutorials/antibody/graphics/antibody.gif) The basic structure of an Antibody was proposed by ["Rodney Porter"](https://en.wikipedia.org/wiki/Rodney_Robert_Porter). It is a 4-shaped molecule with 4 polypeptide chains of which two are long identical heavy chains (H) & two are small identical light chains(l). Hence antibody is represented as H2L3... the two chains are linked by disulphide bonds . ![antibody_isotype-controls.jpg](https://steemitimages.com/DQmZjGcf194LpanJpscvVrtKiKzbVxH4F1EEyn1EQyM4p57/antibody_isotype-controls.jpg) [Source](https://usbio-images.r.worldssl.net/img/Cat2-Images/antibody_isotype-controls.jpg) Antibodies can come in different varieties known as isotypes .There are five antibody isotypes known as IgA, IgD, IgE, IgG, and IgM , each named with an "Ig" as prefix stands for immunoglobulin . References : * [Cells of Immune system](https://www.rndsystems.com/research-area/cells-of-the-immune-system) * [Immunology](https://en.wikipedia.org/wiki/Immunology) * [Antibodies](https://en.wikipedia.org/wiki/Antibody) * [Soluble mediators in Immunity](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4173950/) * [Lines of defence](https://www.iia.org.uk/resources/audit-committees/governance-of-risk-three-lines-of-defence/) * [Antigen presenting cells](https://en.wikipedia.org/wiki/Antigen-presenting_cell) * [Structure of Antibody](http://www.biology.arizona.edu/immunology/tutorials/antibody/structure.html) * [Lymphocytes](https://www.healthline.com/health/lymphocytes) ![upvote.gif](https://steemitimages.com/DQmRhDtjokAZnGKi4QwheqksKTFo6m4fsjMYsNNrsitC1xk/upvote.gif)