protective membranes line all body cavities and organs exposed to the exterior surface
Surface membranes provide mechanical barriers to pathogens
Some have structural modifications and produce secretions that enhance their defensive effects
Examples: the skin’s acidity, lysozyme, mucus, keratin, and ciliated cells
Internal Innate Defenses: Cells and Chemicals
the innate cellular and chemical defenses provide the body’s second line of defense
Phagocytes (macrophages, neutrophils, etc) engulf and destroy pathogens that breach epithelial barriers. This process is facilitated when opsonins (antibodies or complement to which the phagocyte’s receptor’s can bind) attach to the pathogen’s surface.
Natural Killer Cells
Natural killer cells are large granular lymphocytes that act nonspecifically to kill virus-infected and cancerous cells
Inflammation: Tissue Response to Injury
The inflammatory response prevents the spread of harmful agents, disposes of pathogens and dead tissue cells, and promotes healing. Exudate forms; protective leukocytes enter the area; fibrin walls off the area, and tissue repair occurs
Cardinal signs of inflammation: swelling, redness, heat, pain. These result from inflammatory chemicals that induce vasodilation and make blood vessels more permeable.
Interferons are a group of related proteins synthesized by virus-infected cells and certain immune cells that prevent viruses from multiplying in other body cells
Activation of complement (a group of plasma proteins) on the membrane of a foreign cells promotes phagocytosis of that cell, enhances inflammation, and sometimes causes lysis of the target cell
Fever enhances the body’s fight against pathogens by increasing metabolism, which speeds up defensive actions and repair processes, and by increasing metabolism, which speeds up defensive actions and repair processes, and by promoting the liver and spleen to sequester iron and zinc needed for bacterial multiplication
The adaptive immune system recognizes something as foreign and acts to immobilize, neutralize, or remove it. The adaptive immune response in antigen-specific, systemic, and has memory. It provides the body’s third line of defense
Antigens are substances capable of generating an immune response
Complete antigens and haptens
Complete antigens have both immunogenicity and reactivity. Incomplete antigens or haptens must combine with a body protein before becoming immunogenic
Antigenic determinants are the portions of antigen molecules that are recognized as foreign. Most antigens have many sites.
Self-antigens: MHC proteins
Major histocompatibility complex (MHC) proteins are membrane-bound glycoproteins that mark our cells as “self”
Lymphocytes arise from hematopoietic stem cells of the bone marrow and are educated to develop immunocompetence and self-tolerance. T cells are educated in the thymus and provide cellular immunity. B cells are educated in the bone marrow and provide humoral immunity. Immunocompetence is signaled by the appearance of antigen-specific receptors on the surface of the lymphocyte. Immunocompetent lymphocytes seed the secondary lymphoid organs, where the antigen encounter occurs, and circulate between the blood, lymph, and lymphoid organs. When naive lymphocytes encounter their antigen, clonal selection, proliferation and differentiation occur. Most of the clone members become effector cells, but some become memory cells.
In both B and T lymphocytes, antigen receptor diversity is generated by shuffling gene fragments
Antigen-presenting cells (APCs) include dendritic cells, macrophages and B lymphocytes. They internalize antigens and present antigenic determinants on their surfaces for recognition by T cells
HUMORAL IMMUNE RESPONSE
Activation and differentiation of B cells
When B cells are activated, most of the clone members become effector cells called plasma cells, which secrete antibodies. This is the primary adaptive immune reponse.
Other clone members B cells, capable of mounting a rapid attack against the same antigen in subsequent encounters (secondary immune response) The memory B cells provide humoral immunological memory
Active humoral immunity is acquired during an infection or via vaccination and provides immunological memory. Passive immunity is conferred when a donor’s antibodies are injected into the bloodstream, or when the mother’s antibodies cross the placenta. Its protection is short-lived; immunological memory is not established.
The antibody monomer consists of four polypetide chains. Two heavy and two light. Each chain has both a constant and a variable region. Constant regions determine antibody function and class. Variable regions enable the antibody to recognize its specific antigen.
Five classes of antibodies exist: IgM, IgA, IgD, IgG, and IgE. (MADGE)They differ structurally and functionally
Monoclonal antibodies are pure preparations of single antibody type useful in diagnostic tests and treating some cancers
CELLULAR IMMUNE RESPONSE
MHC proteins and antigen presentation
MHC proteins present antigens to T cells. Class I MHC proteins are found on all nucleated cells, but class II MHC proteins are found only on APCs
Activation and differentiation of T cells
Immunocompetent CD4 and CD8 T cells are activated by binding to an antigen-MHC complex on the surface of an APC. A co-stimulatory signal is also essential. The resulting clone members differentiate into either the appropriate effector T cells that mount the primary immune response (ie helper or cytotoxic T cells) or memory T cells
The immune response is enhanced by cytokines such as interleukin 1 released by macrophages and interleukin 2, gamma interferon, and others released by activated T cells
Roles of specific effector T cells
Helper T cells are required for full activation of most b and T cells, activate macrophages, and release essential cytokines. Cytotoxic T cells directly attack and kill infected cells and cancer cells. Regulatory T cells help to maintain tolerance
Organ transplants and prevention of rejection
Cell-mediated responses reject grafts and foreign organ transplants unless the recipient is immunosuppressed. Infections are major complications in such patients
HOMEOSTATIC IMBALANCES OF IMMUNITY
Immunodeficiency diseases include acquired immune deficiency syndrome (AIDS), caused by HIV, and SCID. Overwhelming infections are fatal because the immune system is unable to combat them.