Components of the Specific Defense System
- Identify, destroy, remember
- Cellular components – B cells and T cells – lymphocytes which are white blood cells
- Humoral (antibody-mediated response) defends against extracellular pathogens by binding to antigens and making them easier targets for phagocytes and complement proteins
- Cell mediated immune response – defends against intracellular pathogens and cancer by binding to and lyzing the infected cells or cancer cells
Humoral or antibody-mediated response – termed anti-body mediated because B cells produce antibodies and Humoral because antibodies are released into the bloodstream
B cells – are produced and mature in the bone marrow – they possess a protein on the B cells outer surface known as the B cell receptor (BCR) which allows them to bind to a specific antigen Plasma B cells also known as plasma cells, plasmocytes, and effector B cells– they produce antibodies Memory B cells – ready for the next invasion
- B cell comes into contact with antigen on microbe
- it attaches to the antigen and becomes an antigen-presenting B cell with antigen-MHC complex
- Helper T cell that binds to the complex
- Helper T secretes interleukin that stimulates mitosis in B cells so they multiply
- Some B cells mature into plasma cells and other become memory cells
- The plasma cells produce antibodies also called immunoglobins – proteins which attach to the antigens
Antibodies can clump microbes for destruction, mark microbes for destruction by phagocytes, activate complement proteins that rupture/lyse microbe cell membranes or infected host cells
Antibody Targets and Functions
- Complement fixation: Foreign cells are tagged for destruction by phagocytes and complement fixation Immune complex formation exposes a complement binding site on the C region of the Ig and Complement fixation results in cell lysis.
- Neutralization: immune complex formation blocks specific sites on virus or toxin & prohibit binding to tissues (antibodies block active sites on viruses and bacterial toxins so they can no longer bind to receptor cites on tissue cells and cause injury)
- Agglutination: cells are cross-linked by immune complexes & clump together
- Precipitation: soluble molecules (such as toxins) are cross-linked, become insoluble, & precipitate out of the solution
- Inflammation & phagocytosis prompted by debris
Antigen-Antibody Complex – Functions
|IgA||Antibodies are dimmers – contain two Y shaped structures. Found in mucosal areas, such as the gut, respiratory tract and urogenital tract. Also found in saliva, tears, and breast milk. They attack microbes and prevents colonization by pathogens before they reach the blood stream so it is most important antibody in local immunity|
|IgD||Functions mainly as an antigen receptor on B cells that have not been exposed to antigens. It has been shown to activate basophils and mast cells to produce antimicrobial factors.|
|IgG||In its four forms, provides the majority of antibody-based immunity against invading pathogens. It makes up about 75 % of all human antibodies and is the body’s major defense against bacteria. The only antibody capable of crossing the placenta to give passive immunity to fetus. It is the most versatile of antibodies because it carries out functions of the other antibodies as well.|
|IgE||Binds to allergens and triggers histamine release from mast cells and basophils, and is involved in allergy. Also protects against parasitic worms.|
|IgM||Expressed on the surface of B cells and in a secreted form with very high avidity. Eliminates pathogens in the early stages of B cell mediated (humoral) immunity before there is sufficient IgG.|
Memory B cells are stimulated to multiply but do not differentiate into plasma cells; they provide the immune system with long-lasting memory.
Cell-mediated immune response (within the cell) – does not involve antibodies but rather involves the activation of phagocytes, antigen-specific cytotoxic T-lymphocytes, and the release of various cytokines in response to an antigen
- T cells – are produced in bone marrow but mature in the thymus glandT cells contribute to immune defenses in two major ways: some direct and regulate immune responses; others directly attack infected or cancerous cells.
- Helper T cells – assist other white blood cells in the immunologic process including maturation of B cells into plasma cells and memory B cells and activation of T cells and macophages
- Cytotoxic T cells – sometimes called killer T cells destroy virally infected cells and tumor cells and play a role in transplant rejection
- Memory T cells –antigen-specific T cells the persist long-term after an infection has been resolved that will provide memory of past infection and earlier defense for new infection
- Regulatory T cells – formally called suppresser T cells maintain balance by shutting down T-cell mediated immunity toward the end of an immune reaction – they are a self check built into the immune system to prevent excessive reactions. They play a key role in prevent autoimmunity.
- Antigens are proteins or carbohydrate chain of a glycolprotein within in plasma membrane that the body recognizes as nonself
- The antigens on the cell membrane of the target or invader cell are recognized
- MHC (a protein marker on body’s cell) binds to the antigen of the foreign cell forming an MHC complex
- The MHC complex alerts the T cells about an invasion, macrophage, virgin B cell or cell infected by a microbe that displays the antigen on its membrane
- The MHC complex activates the T cell receptor and the T cell secretes cytokines
- The cytokines spur the production of more T cells
- Some T cells mature into Cytotoxic T cells which attack and destroy cells infested with viruses or cancerous cells
Cytotoxic T cells or Killer T cells (NKT) share the properties of both T cells and natural killer (NK) cells. They are T cells with some of the cell-surface molecules of NK cells.
The kills cancer cells, cells that are infected (particularly with viruses), or cells that are damaged in other ways -They have storage granules containing porforin and granzymes (proteins which perforates the cell membrane of the cell to be destroyed allowing water & salts to enter and rupture the cell).
They and are implicated in disease progression of asthma and in protecting against some autoimmune diseases, graft rejection, and malignant tumors
- Other T cells mature into Helper T cells which regulate immunity by increasing the response of other immune cells
- Helper T cells secrete cytokines (messenger molecules) when exposed to antigens that causes more Helper T cells to be cloned, B cells to make antibodies and macrophages to destroy cells by phagocytosis
- AID’s virus attacks to Helper T cells so it inactivates the immune system
- Regulatory T cells will shut down T-cell mediated immunity when things are under control
- Memory T cells persist sometimes for life and protect in case of re-infection