Microbiology/Immunology lect.6 Dr. Mohammed A.K.AL-SA ADI
Lymphoid Organs :
Lymphoid organs are classified as: 1-Primary(generative) lymphoid organs: Where lymphocytes first express antigen receptors and attain phenotypic and functional maturity. The major primary lymphoid organs are thymus and bone marrow. 2-Secondary(peripheral) lymphoid organs: Where lymphocytes respond to foreign antigens. These organs include spleen, lymph nodes, and mucosa-associated lymphoid tissues(MALT).
Thymus:
The thymus is the site of T cell maturation and education (learn to discriminate between self and non-self antigens).The lymphocytes (thymocytes) are T cells at various stages of maturation .The most immature T cells enter the thymic cortex through the blood vessels .Maturation begins in the cortex ,then thymocytes migrate toward the medulla ,so that the medulla contains mostly mature T cells. After puberty, Thymus atrophies.
Bone Marrow:
Bone marrow is the site for the generation of all circulating blood cells in the adult and is the site for B cell maturation and education.
Lymph Nodes:
Lymph nodes are the organs in which adaptive immune response to lymph-borne antigens are initiated. A lymph node consists of an outer cortex and an inner medulla .The aggregates of cells (follicles)are present in the cortex ,and there are two types of follicles :1-primary follicles without germinal center enriched with mature but naïve B cells, 2-secondary follicles with germinal center and mature proliferative B cells with high affinity to produce Ab and memory cells. APCs are located in the areas around the follicles . .Lymph nodes are interposed along lymphatic vessels and act as filters that clear the lymph at different sites before it reaches the blood.
Spleen:
The spleen is an important filter for blood .Macrophages of red pulp clear the blood from microbes and other foreign particles ,and the spleen is the major site for the phagocytosis of Ab-coated (opsonized) microbes .Individuals lacking a spleen (splenoctomized)are extremely susceptible to encapsulated infections such as pneumococci and meningococci because such microbes are normally cleared by opsonizatoion and phagocytosis .This function is defective in the absence of the spleen(splenoctomy).
MALT:
The immune response mediated by MALT is characterized by the production of high level of S-IgA .So the main function of MALT is the local defense against foreign substances at mucosal surfaces.
MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) :
The preparation of cell-associated Ag for recognition by T cells is performed by specialized proteins that are encoded by genes in a locus called MHC. There are two main classes of MHC gene products called MHC I and MHC II, which sample different pools of protein Ag cytosolic (intracellular)Ag and extracellular Ag. MHC I molecules present Ag to CTLs(CD8 T cells) ,whereas MHC II present Ag to TH cells(CD4 T cells).The human MHC molecules are also called HLA(human leukocyte antigens) and the gene locus is present on chromosome ( 6) . Thus, HLA typing should be carried out prior to each transplantation to avoid the rejection.
Cytokines
Many critical interactions among cells of the immune system are controlled by soluble mediators called cytokines, which are diverse group of intercellular signaling peptides and glycoproteins with MW (6000-60000)D and concentration 10-9 _ 10-15 M. Cytokines represent "the hormones of immune system". They act by binding to specific surface receptors on target cells. Unlike endocrine hormones ,they are not produced by specialized glands and secreted into the circulation ,but rather are produced locally by a variety of cells and tissues.