LYMPHOID TISSUE I

Histology 2016-2017
Department of Anatomy &Histology: Dr.Rajaa Ali
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LYMPHOID TISSUE I
INTRODUCTION
• Lymphoid system (immune system) consists of tissues and organs mainly made of lymphocytes, which protect the internal environment of the body against invasion of microorganisms by producing specific immune response. This immune response is activated only when the first two lines of defense, described below, fail to prevent the entry of microorganisms.
• The first line of defense is provided by the surface epithelium constituted by the epidermis of skin that covers the body and the mucous membrane that lines the hollow visceral tracts which are exposed to the external environment.
• The antibacterial substances present in various secretions (tears, saliva, etc.) on the mucous membrane and the acidic environment present in the stomach and vagina inhibit the growth and entry of microorganisms into the underlying tissue.
• The second line of defense comes into effect when there is breach of the epithelial surface due to injury (abrasion, burning, etc).
• The second line of defense is constituted by phagocytic cells, namely, macrophages and neutrophils present in the connective tissue (lamina propria, dermis). These cells exert a nonspecifi c cellular response by destroying the pathogenic bacteria by phagocytosis or by producing an antiviral substance, interferon, against multiplication of virus within cells.

• The site of pathogenic insult evokes a tissue reaction called infl ammation.
• The third line of defense is the specifi c immune response which gets activated when both the above mentioned lines of defence fail to check the invasion of pathogenic organisms. The specifi c immue response is evoked by lymphocytes.
• Lymphocytes are of two types, namely, B and T. They are derived from common stem cells in the bone marrow. Though they are morphologically similar, they are functionally different.
• B lymphocytes mature and become immunocompetent in the bone marrow, whereas T lymphocytes mature and become immunocompetent in thymus and migrate to other peripheral lymphoid organs.
• They express their defense mechanisms in two main ways in response to antigen either separately or often together.
• The defense mechanism mediated by B lymphocytes is called humoral immune response, in which the immunocompetent B lymphocytes encounter a specifi c antigen and become activated. These activated B lymphocytes proliferate and differentiate into plasma cells. The plasma cells secrete specifi c antibodies (immunoglobulins) into the blood and lymph against that
• particular antigen.