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The lymphatic system is a network of
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tissues, organs and vessels that helps
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maintaining fluid balance that include
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water balance and electrolyte balance.
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Defends the body against infection or
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immune response and also helps in
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dietary fat absorption. In this video,
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we are going to discuss the difference
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between primary lymphoid organs and
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secondary lymphoid organs. The lymphoid
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organs are classified into primary and
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secondary based on their roles in the
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development and function of lymphocytes.
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Let us understand with examples. The
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primary lymphoid organs are bone marrow
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and thymus. So this is a bone marrow.
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This is a site where bell production and
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maturation takes place. And the second
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In thymus T- cell maturation takes
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place. T cells are produced in bone
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marrow but maturation takes place in
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thymus. So primary lymphoid organs are
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sites where lymphosytes are produced and
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mature. The secondary lymphoid organs
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include lymph nodes, spleen, mucosa
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associated lymphoid tissue like tonsils,
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pis patches, appendix etc. These are the
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sides where mature lymphosytes become
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functional and initiate immune
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responses. Difference number two in
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primary lymphoid organs like bone marrow
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and thymus these lymphosytes
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and also under goes differentiation and
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maturation to form fully functional
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lymphoid cells. Whereas in secondary
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lymphoid organs like spleen and lymph
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nodes these lymphosytes are activated
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and starts division upon encountering
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with an antigen. Suppose this is a B
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cell and this is an antigen. This B cell
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has specific receptors specifically
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designed to bind to epitops of an
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antigen. So upon binding of this B cell
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to antigenic epitop, this B cell is
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activated and starts dividing forming
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clones of B cells with the same
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specificity. In secondary lymphoid
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organs, activation and proliferation of
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lymphosytes takes place upon
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encountering an antigen. Difference
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number three in primary lymphoid organs
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like thymus and bone marrow there is no
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direct contact with foreign antigens. As
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you see production and maturation takes
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place in bone marrow and thymus without
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the involvement of foreign antigens.
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Only self antigens are present in these
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sites. Whereas secondary lymphoid organs
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there is direct contact with foreign
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antigen that stimulates immune response
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while lymphosy activation. As we
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mentioned earlier, this B cell can bind
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to epitops of antigens. Upon binding to
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the epitops of antigen, this B cell is
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activated and stimulates immune response
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by the activation of this lymphocy. Now
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regarding the function primary lympoid
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organs the primary function is
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hematopois or reduction of all blood
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cells takes place in bone marrow both
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lymphosytes and other blood cells. The
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second function is imunoglobilin TCR
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gene rearrangement. So that happens this
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B cell precursors undergo a WDJ
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rearrangement or re combination to
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generate diverse B cell receptors that
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is specific against epitops of different
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antigens and third function is central
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tolerance which is a fundamental
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imunological process that eliminates
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self-reactive lymphosytes during their
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development preventing autoimmune
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response. So as you see here in this
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case this mature T- cells has high
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affinity for self antigens so that these
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T- cells are selected and that is
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destroyed by apoptosis or program cell
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death. This mechanism ensures immune
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cells do not attack bodisol tissues. The
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functions of secondary lymphoid organs
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include antigen presentation.
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Suppose this is an APC antigen
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presenting cells like dendritic cells or
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macrofasages that presents antigen to a
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lymphocy like B cell or T cell. Upon
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activation here you can see there is B
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cell 1, B cell 2 and B cell 3. BL 2 has
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a receptor that is that can bind to this
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antigen. So this particular B cell 2 is
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activated and this single lymphocy upon
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binding to a specific antigen divides or
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proliferates rapidly producing
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genetically identical cells or clones
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and this is called as clonal expansion
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or that particular B cells is selected
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to divide and proliferate to form large
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number of B cell with a specific
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specificity targeting this particular
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antigen. And the next step is adaptive
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immune response. This particular B cell
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that is B cell 2 lymphocytes under goes
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differentiation to form plasma cells and
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memory B cells. These plasma cells are
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involved in secretreting antibodies that
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has specificity towards this antigen and
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these antibodies bind to this antigen
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finally ensuring its clearance from the
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system. Memory B cells are also produced
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and that is involved in imunologic
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memory. When the antigen comes for a
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second time, there will be an enhanced
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immune response because of the presence
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of memory B cells. For more details, you
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can watch our videos on this topic. You
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can check out our imunology video
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simplified versions of different aspects
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of our immune system. So these are some
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of the titles. Hope you are benefited
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from this video. Take care, stay
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blessed. Thank you so much. You are with
