Organs of Immune System:
Organs of immune system found throughout the body and are referred to as lymphoid organs comprise tonsils and adenoids, lymph nodes, thymus, lymphatic vessels, bone marrow, Peyer's patches, spleen, and appendix. These organs take part in growth, development and sending of lymphocytes to wherever body's defense system is compromised. There are 2 classes of lymphocytes - B and T lymphocytes and this classification is based on where they mature. B lymphocytes complete the maturation in bone marrow while T lymphocytes migrate to thymus where they multiply and mature in cells which become immunocompetent. T cells become educated to be able to differentiate self cells from non-self cells.
Components of the Immune System:
Immune system gives defense against infectious micro organisms, and also non infectious foreign substances may also elicit the immune reaction. Cells of immune system originate from bone marrow and cells remain there till maturity. Principal components of immune system are lymphocytes, accessory cells and effector cells. They migrate to tissues and circulate in blood in the specialized system of vessels called as lymphatic system. Red blood cells, platelets and white blood cells are derived from precursor cells called hematopoietic stem cells found in bone marrow. Hematopoietic stem cells are said to be pluripotent as they provide rise to different kinds of blood cells. Myeloid precursor is also significant in immune response and they give rise to granulocytes, macrophages, Dendritic cells and mast cells.
The Lymphoid Cell:
Lymphocytes are derived from bone marrow like blood cells and go through complex maturation procedure before they are able to obtain their characteristics. Lymphocytes are the only cells in body which are able to recognize and differentiate different antigenic determinants. Small lymphocytes which haven't encountered the antigen before are known as naive lymphocytes and on activation by antigen they increase in size and are then known as large lymphocytes or lymphoblasts. Lymphocytes are heterogeneous and their subsets have functions and protein products but they are same in morphology. B cells or lymphocytes are liable for antibody production and are so called as they develop in bone marrow of mammals while in birds they are present and reach maturity in the organ known as bursa of Fabricus.
These cells, as earlier defined comprise of granulocytes, dendritic, macrophages and mast cells of immune system. Granulocytes or polymorphs comprising of neutrophils, esinophils, basophils, monocytes etc are so called as they have granules having acidic and alkaline phosphatases, defensins and peroxidases. These latter molecules are needed for clearing of pathogen. Macrophages are long lived and function to swallow up any pathogen using phagocytosis on passing membrane barrier. Once pathogen is engulfed, a phagosome is formed and lysosomes secrete their enzyme leading to destruction of pathogen.
Macrophages secrete cytokines whose function comprises attracting other cells like short lived neutrophils, and increasing permeability of endothelium and production of neutrophils in bone marrow. Dendritic cells take up antigens and show them for identification by lymphocytes. Immature dendritic cells migrate from blood to stay in tissues showing both phagocytic and macropinocytic activities by taking up large quantities of fluid, but once they come across the pathogen they quickly mature and migrate to lymph nodes. Mast cells have electron dense granules in the cytoplasm and distinguish in tissues. They stay near blood vessels and on activation, release molecules that influence membrane permeability. They are also involved in eliciting allergic reactions.
Structure of antigenic determinants:
Antigens are substances able to react with antibodies or T cell receptors. Immunogens are substances able of eliciting immune reactions. There are some small molecules that aren't capable of eliciting the immune response unless attached to the macromolecule known as carrier before immunization.
The Features of Antigenic Determinants:
Epitopes may be formed depending on protein folding and covalent structure of molecule. Antigenic determinants of phospholipids and complex carbohydrates are formed by covalent structures while those of some proteins depend on covalent structure and others on tertiary structure of protein. In case of nucleic acids and proteins, antigenic determinants depend on non covalent folding of molecule. Also, spatial arrangements may influence antigen binding such that when epitopes are well separated from each other two antibody molecules could bind to same protein without having any effect on each other. Neo-antgenic determinant may be formed when there is protein modification like phosphorylation and proteolysis leading to change in covalent structure of protein. New epitopes are consequently produced that could be recognized by antibodies.
Structural Basis of Antigen Recognition:
The study of protein lysozyme illustrated that structure of antigenic determinant recognized by antibody binding site has three principal characteristics namely: should be large approx 750 ?2 in area, formed by two segments of amino acids which are extensively separated in primary structure but adjacent in its three dimensional structure and at last exposed on surface of antigen. Size of antigenic determinants thus would depend on the number of CDRs it has contact with. Two structural antigenic determinants have been proposed, continuous and non continuous determinants. Residues in contact with antibody are all housed within the single segment of amino acid sequence of antigen in continuous determinants while in non continuous determinants, amino acid residues are far apart but come together as a result of protein folding in native conformation.
Kinds of Antigens:
B cells respond to three kinds of antigens. Antigens could be T-independent or T- dependent. T-independent antigens can generate antibodies on stimulation by B cells without involvement of T cells e.g. polysaccharides. When B cells are stimulated responses received from these antigens are different from other antigens. T- Independent antigens are characterized by having polymeric structure, resist degradation and have skill to polyclonally activate other cells. Though, T-independent antigens may also be categorized in Types 1 and 2 based on their skill to polyclonally activate B cells, former activate B cell clones while latter don't. T dependent antigens need the help of helper T cells to stimulate antibody production by B cells.
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