Introduction
Our immune system functions to resist infections by specific pathogens. Whether inherited, acquired, or induced, immunity in all forms builds the protection mechanism of the body. The functioning of the immune system depends on a wide variety of substances, though; the most remarkable ones include lymphocytes, especially B cells and T cells, and antibodies. The aim of the B cell is to produce antibodies against foreign substances, viruses, or bacteria, etc, whereas T cells play an important role in the cell-mediated response. “There are three types of T cells: cytotoxic, helper, and suppressor T cells.” (Last, John) Cytotoxic T cells produce a cell-mediated immune response and destroy virus-infected cells.
On the other hand, helper T cells activate the antibody, as well as, the cell-mediated immune responses. Suppressor T cells deactivate T cells and B cells when needed, and thus prevent the immune response from becoming too intense. Antibodies are Y-shaped immunoglobulins that are circulating in the blood. They are composed of proteins and their function is to fight against foreign substances called antigens.
Main body
According to the dictionary definition, Antigens are defined as molecules that generate an immune response in the body. Antigens can be composed of proteins, polysaccharides, or are found as conjugates of lipids and proteins, known as lipoproteins, and lipids with polysaccharides, known as glycolipids. Though, any substance that can elicit an immune response and forces it to produce antibodies can be labeled as antigen. Living foreign bodies, including viruses, bacteria, and fungi, causing disease and infection, plus substances, like dust, pollen, grains, or dust that produce allergic reactions. Antigens producing allergic reactions are known as allergens.
Antigens can also be produced by the body itself; an example of such antigen includes cancer cells. This mechanism is used when an organ transplant or a blood transfusion is done without tissue typing and cross-matching. The immune system attempts to differentiate its “self” from foreign substances and rejects the organ transplant or a blood transfusion. (Antibody and Antigen World of Microbiology and Immunology Summary, n.d.)
There are many substances that rarely produce antigenic properties. These substances include nylon, plastic, or Teflon and due to their non-antigenicity, they are used as transplants for artificial blood vessels, pacemakers in the heart, and also needles used in hypodermic syringes. These substances rarely trigger an immune system response and hence are safe to be used and can be beneficial to human health.
However, there are substances that are highly antigenic, for example, “the venoms from insect and reptile bites, solvents, formalin, and asbestos.” Infections, whether bacterial, viral, or fungal, also usually elicit an immune response by activating certain antibodies. (Antibody and Antigen World of Microbiology and Immunology Summary, n.d.)
There is another type of antigen that is found in the tissue cells of organ transplants. These antigens are called human leukocyte antigens (HLA). For example, when a kidney is transplanted; its surface cells contain antigens that the new host begins to reject. There are four types of human leukocyte antigens (HLA) that are further subdivided into smaller groups. HLA compatibility and tissue matches are performed in order to avoid organ rejection. As a precaution, certain drugs are also given to suppress immunity and decrease the chances of tissue transplant rejection. These drugs act by decreasing, as well as, controlling the production of helper T cells and the number of antibodies.
ABO antigens are surface antigens that are present in the red blood cells. These antigens pose a problem during blood transfusions. Before any blood transfusion can be performed, blood typing is performed to ensure that only that blood is used in a transfusion that is compatible. Type A blood has antigen A and type B has antigen B. when both A and B antigens are present the blood typing is said to be of type AB, whereas Type O blood has no antigens. For a successful blood transfusion of a patient with type A blood, it would require either type A or O. Type B and AB would be rejected. Type B blood is compatible with a B donor or an O donor. O is considered to be the universal donor because has no antigens and hence is compatible with all blood types. Since AB can accept A, B, AB, or O is called the universal recipient. (Levinson, 2006)
Another perspective where antigens are important is maternal alloimmunization or isoimmunization. “This occurs when a woman’s immune system is sensitized to foreign erythrocyte surface antigens, stimulating the production of immunoglobulin G (IgG) antibodies.” Maternal sensitization occurs through blood transfusion or fetomaternal hemorrhage. Fetomaternal hemorrhage is the passage of fetal red blood cells via the placenta and is “associated with delivery, trauma, spontaneous or induced abortion, ectopic pregnancy, or invasive obstetric procedures.” (Tran, 2008)
There are several different kinds of antigens that can cause maternal alloimmunization and fetal hemolytic disease. “Among those the Rhesus (Rh) blood group system is the most common which comprises of the c, C, D, e, and E antigens.” severe cases of the hemolytic disease are produced by the antigen D. Hemolysis of fetal erythrocytes and anemia can produce life-threatening outcomes for the fetus, including hydrops fetalis. (Tran, 2008)
Rhesus disease, or erythroblastosis fetalis, affecting newborn babies is another antigenic blood condition. The cause of Rhesus disease, or erythroblastosis fetalis, is the incompatibility between the fetus and the mother’s erythrocytes, of Rh factors. If the baby’s blood is Rh-positive and it is transferred into the Rh-negative mother, then this will result in the production of antibodies against Rh-positive red blood cells. In the subsequent pregnancy, these antibodies are then passed from the mother to the fetus against fetuses’ Rh-positive blood. Thus, the fetus will suffer from Rh disease if the fetus is Rh-positive. (Tran, 2008)
Antigens are classified according to their site of origin as Exogenous antigens, Endogenous antigens, Autoantigens, and Tumor antigens. Exogenous antigens are antigens that enter the body from the outside, for example by inhalation through the nose, ingestion with food, or injection directly through the skin. These antigens are then taken up by the antigen-presenting cells (APCs) either by endocytosis or phagocytosis and processed into fragments. Antigen-presenting cells (APCs) are comprised of phagocytic cells and B lymphocytes. Phagocytic cells include dendritic cells and macrophages.
The B lymphocytes produce antibodies against the antigen.APCs then use the class II histocompatibility molecules present on their surface. The Class II histocompatibility molecules are composed of three molecules. Two of them are transmembrane polypeptides, whereas the third molecule is lodged in between the groove formed by the two transmembrane polypeptides. For Class II histocompatibility molecules to function properly, it is essential that all three components of this complex are transported into the endoplasmic reticulum. The groove is occupied by a temporary protein called the invariant chain (“Ii”).
This temporary protein is used to transport these antigenic peptides into the endoplasmic reticulum. The two chains of the class II molecule are transported into the membrane of the endoplasmic reticulum. Bond is formed between a molecule of invariant chain and the membrane, in their groove. “This trimolecular complex is then transported through the Golgi apparatus and into vesicles called lysosomes.” (Antigen Presentation, n.d.)
The antigen is broken down into small fragments followed by the digestion of the invariant (Ii) chain. During this process, the grooves become free for the antigenic fragments to occupy them. These vesicles then move into the plasma membrane exposing the complex at the cell’s surface. The complex can bind by either a T cell with a receptor (TCR) or CD4 receptor. The T cell’s function is to form a bond between the peptide and adjoining areas of the histocompatibility molecule. On the other hand, the CD4 molecules bind the CD4 receptor that is found on all class II histocompatibility molecules. The activated helper T cells (CD4+), as well as the specific T cells for the peptide: MHC complex, secrete cytokines. Cytokines are proteins that activate cytotoxic T lymphocytes, B cells secreting antibodies, and macrophages.
Endogenous antigens are antigens that are produced inside the cell. Endogenous antigens are formed as a result of normal cell metabolism, viral infection, or intracellular bacterial infection. These fragments, together in a complex with MHV class I molecules, are presented on the cell surface.
Class I histocompatibility molecules include transmembrane proteins that are expressed at the surface of the. As with all transmembrane proteins, class I histocompatibility molecules are also synthesized by ribosomes that are attached to the rough endoplasmic reticulum (RER), and then they are assembled within its lumen. “There are three subunits in each class I histocompatibility molecule: the transmembrane polypeptide (called the “heavy chain”), the antigenic peptide, and beta-2 microglobulin.” (Antigen Presentation, n.d.)
The cytotoxic CD8+ T cells recognize these complexes and the release of various toxins by the T cells. These toxins cause lysis or apoptosis of the infected cell.
An autoantigen is a normal protein or complex of proteins that are recognized by the immune system as foreign substances or antigens. This is observed more commonly in patients who are suffering from some specific kind of autoimmune disease.
Conclusion
Lastly, Tumor antigens are antigens that are found on the surface of tumor cells. Tumor antigens are presented by both MHC I and/or MHC II molecules. Moreover, these antigens are also presented by tumor cells in most instances. The cytotoxic T lymphocytes recognize these antigens and attempt to destroy the tumor cells before they proliferate or metastasize.
Antigens are macromolecules that produce an immune response. Though it is made for the protection mechanism of the body, it can also sometimes be beneficial. Whether the antigen is producing disease or they are used for cure and treatment, a lot more research is needed to fully understand the antigenic properties of the antigens.
References
Antigen Presentation. Web.
ANTIGEN PRESENTATION. 2008. Web.
Gale, Thomson. (2006) Antibody and Antigen World of Microbiology and Immunology. Thomson Corporation. Web.
Klein-Gitelman, Marisa. (2006). Systemic Lupus Erythematosus. Web.
Levinson, Warren. (2006) Review of Medical Microbiology and Immunology. Lange Medical Books/McGraw-gill Medical Publishing Division.
McDevitt, Hugh. (2004) Specific antigen vaccination to treat autoimmune disease. Web.
Tran, Susan. (2008) Erythrocyte Alloimmunization and Pregnancy. Web.