The Immune System. Organs of the immune system: Bone marrow, thymus, spleen, lymph nodes and mucosa- associated lymphoid tissues. During hematopoiesis, bone marrow-derived stem cells differentiate into either mature cells or into precursors of cells that migrate out of the bone marrow to continue their maturation elsewhere, for example, maturation of T lymphocytes occurs in the thymus. The spleen acts as an immunologic filter of the blood, whereas the nodes filter the tissue fluids (i.e. lymph). The mucosa-associated lymphoid tissues are aggregates of lymphoid tissues near the mucosal surfaces. Cells of the immune system: Leukocytes (white blood cells) consist of: (a) Polymorphonuclear granulocytes (PMN): Neutrophils, eosinophils and basophils; (b) Monocytes: can be differentiated into macrophages; (c) Natural killer cells; (d) B lymphocytes: produce antibodies and (e) T lymphocytes: T helper cells, suppressor T cells and cytotoxic T cells. Lymphocytes and mononuclear phagocytes play a central role in the immune response. Immune response: Antigen presenting cells (APCs) are cells that capable of processing an antigen and presenting part of it onto the MHC II where it can interact with the appropriate immune cell receptors. Dendritic cells, macrophages and B cells are the main APCs for T cells, whereas follicular dendritic cells are the main APCs for B cells. APCs and B or T cells communicate together, either directly or via cytokines, to initiate an immune response. Antigen processing and presentation signal these cells to proliferate and secrete antibodies (B cells), cytokines (CD4+), or become activated to kill cells expressing the antigen presented by the APC (CD8+). The antibodies secreted by B cells can directly bind to that antigen, which accelerate the clearance by the PMN or macrophages. The antibodies may also initiate the complement destruction cascade by attracting the serum protein to bind to the immobilized antibodies that are bound to the antigen, and thereby aiding in the phagocytosis process and elimination of the immune complex.
