Anything that causes an immune response is called an antigen. An antigen may be relatively harmless, such as grass, pollen or animal dander, or harmful, such as the flu or HIV virus.
Disease-causing antigens are called pathogens. The immune system is designed to protect the body from pathogens.
In the human body, the immune system begins to develop in the embryo. The immune system starts with hematopoietic (from the Greek language meaning "blood making") stem cells. These stem cells differentiate into the major components of the immune system (monocytes, lymphocytes and granulocytes), present in the bone marrow, the lymphatic system, and to a lesser degree, in the thymus gland. These stem cells also differentiate into cells in the blood that are not involved in immune function, such as erythrocytes (RBC's) and megakaryocytes (for blood clotting). Stem cells continue to be produced and differentiate throughout the life cycle.
INNATE VS ADAPTIVE IMMUNITY
The immune system is typically divided into two categories: innate and adaptive.
Innate immunity refers to nonspecific defense mechanisms that come into play immediately or within hours of an antigen's appearance in the body. These mechanisms include physical barriers such as skin, chemicals in the blood, and immune system cells that attack foreign cells in the body. The innate immune response is activated by chemical properties of the antigen.Newborns are possessed of certain immunities, or pathogenic recognitgion properties, which have been passed on in utero from the mother. These generally are present 2-6 weeks after birth.
Adaptive immunity refers to antigen-specific immune response. The adaptive immune response is more complex than the innate. The antigen first must be processed and recognized. Once an antigen has been recognized, the adaptive immune system creates an army of immune cells specifically designed to attack that antigen. Adaptive immunity also includes a "memory" that makes future responses against a specific antigen more efficient. For example, vaccinations activate adaptive immunity by sub-clinical or low-dose exposure to certain diseases.
The Cellular System
T-cells differentiate in the thymus, and have a specific receptor for a protein fragment of antigen.
Cytotoxic or "cell poisonous" T-cells contain a surface protein called CD8 and destroy pathogen infected cells, at times cancer cells, and foreign cells (i.e. transplanted organs).
Helper cells contain a surface protein called CD4 and regulate both the cellular and humoral immune systems. This regulation reduces autoimmunity.
Autoimmune disease/Self immunity diseases include rheumatic fever, rheumatoid arthritis, ulcerative colitis, and myasthenia gravis.
B cells are specialized white blood cells produced in the bone marow. Each B cell contains multiple copies of one kind of antibody as a surface receptor for antigens. The entire populaton of B cells has the ability to specifically bind millions of different antigens.
When the antibody on the surface of a B cell binds to an antigen, the cell can be stimulated to undergo proliferation and differentiation. This process is called clonal selection. The cells produced make the same antibody, but become memory cells and plasma cells. Memory cells insure that subsequent infections by the pathogen receive a more rapid response. Plasma cells secrete large amounts of the antigen-specific antibody. T helper cells, part of the cellular syste,m are usually required for the clonal selection of B cells.
Antibodies secreted by plasma cells form complexes with free pathogens and their toxic products. The complexes can inactivate pathogens and stimulate other innate systems including phagocytic cells and complement cells to eliminate the danger from invasion of pathogens in our extracellular fluids.
Regulation by the Major Histocompatibility complex (MHC)
The major histocompatibility complex is a series of genes that code cell surface proteins which control the adaptive immune response. The system is called H2 in mice and HLA (human lymphocyte antigen) in humans. Class I MHC contains three genes called HLA-A, B, and C; proteins from these genes are expressed on almost all cells. Class II MHC genes are called HLA-DR, DQ, and DP; their proteins are expressed on antigen-presenting macrophages, dendritic cells and B cells.
The function of these proteins is to present fragments of antigens to T cells. The receptor of T cells can only recognize antigen fragments in complex conjunction with MHC proteins.
Following phagocytosis of a pathogen, fragments of the pathogen are complexed or joined with MHC proteins and displayed on the surface of the macrophagic or dendritic cells of the innate immune system. If the cell encounters a dangerous pathogen a co-receptor called B7 is produced. This is a crucial step for activating the adaptive immune response and developing a memory of pathogenic threats. Those helper T cells with a receptor that recongize the antigen fragment of the MHC complex can be stimulated to proliferate and be activated if the co-receptor is also displayed.
The dendritic cell of the innate system is used to initiate adaptive immunity.
Following activation by an antigen presenting cell, the Helper T cell signals B cells and cytotoxic T cells to launch an immune response. This activation of helper T cells occurs in the lymph nodes. Following activation clonal selection occurs , and helper T cells with the correct receptor are activated and proliferate.
Some pathogens can escape antibody detection by infecting or re-replication of cells. Cells to contain pathogenic display antigen fragments on their cell surfaces. Receptors on the surface of cytotoxic T cells (called CD8 cells) can detect the presence of pathogen specific antigen fragments and activate an immune response that leads to the death of the infected cell. Cytotoxic T cells must interact with Helper T cells (CD4 cells) to regulate destruction of infected cells. Helper T cells regulate other cells of the immune system through secretion of molecules called cytokines.
Immune response to self (Autoimmune Disorder)
Tolerance is the lack of an immune response to self. MHC molecules (gene products from MHC genes)undergo cytokinesis to control the body's ability to recognize self and non-self. When T cells mature in the thymus, they are evaluated for their reactivity against self. Normally all T cells which react against self are destroyed. Inappropriate T cells produced in the thymus, about 95% of the total, die and never enter the circulatory system. T cells in the circulatory system must receive signals from normal cellular tissue to survive. When these control systems fail, the immune system can react against the body's own molecules.
Autoimmunity is an immune response against self (auto-->self). Autoimmune diseases occur when the body attacks its own tissues. Rheumatoid arthritis (RA), Myasthenia gravis and SLE(systemic lupus erythematosis) are examples of autoimmune disorders.