MHCs: The Role of Cell Surface Markers

 Immunity = ability to distinguish between "self" and "non-self”  Every cell carries same set of distinctive surface proteins  distinguish you as "self”  Pattern is unique to individual  Set of unique markers on human cells = major histocompatibility complex (MHC).

 Respond against/eliminate foreign agents  Protein toxin molecule  Virus  Infectious bacterium  Lack of MHC = Lack of immune response

 Similarity of patterns of MHC molecules  Skin grafts  Tissue/organ transplants  Organ transplants  Both individuals examined for genetic similarity  Blood group & MHC molecules  10/12 matches  Medicine to lower immune response

 Inherit 2 of each class (MHC I/II)  1 from Mom  1 from Dad  Both sets of Class I or Class II OR sometimes both are expressed on cell surface  Induction of immune response of invader  Class (I/II) expressed on surface depends on type of cell involved

 All nucleated cells = equally express BOTH sets of just MCH I

 Some specialized cells of immune system = express BOTH sets of MCH I AND MCH II  B-lymphocytes  Macrophages  Certain dendritic cells  Capacity to express MHC Class II molecules  known as Antigen-Presenting Cells (APC)

 Can present parts of proteins taken up by the ACP to T lymphocytes.

 First line of defense  Physical barriers of the body Skin Mucous Tears Saliva pH of skin

 2 nd line of defense  White blood cells + inflammatory response Phagocytosis (Neutrophils, macrophages, eosinophils) Inflammation = blood supply increased in injured area (redness, heat) Swelling = histamine released by basophils and mast cells, triggers inflammatory response

 Recognizes SPECIFIC foreign material and destroys it  Antibodies, Killer T cells  Active vs. Passive immunity  Active = develops after recovery disease or vaccination Chicken pox, vaccinations (measles, mumps, rubella)  Passive = passed from one individual to another NOT PERMANENT! Ex: Breast-feeding passes immunities to child

1. Humoral immunity -B Cell activation -Production of antibodies 2. Cell-mediated response - Production of Cytotoxic T Cells

 Antibodies produced  Bind to antigens = easier targets for phagocytes.  Mainly free bacteria, toxins, viruses in body fluids

 T-Cell (Helper T Cell)  Specifically interacts with MHC II shapes  produces cytokines  Cytokines (IL-2) stimulate activated B cells to produce antibodies  Now, anywhere toxin is in system… engulfed by macrophages!

 Secreted proteins + signaling molecules  Interleukin  promote development and differentiation of T & B cells  Interleukin-1 (IL-1)  Inflammatory response (fever)  Macrophage secretes IL-1 after presenting antigen  IL-1 + presented antigen  Activates helper T Cell to produce IL-2

 Secreted proteins + signaling molecules  Interleukin  promote development and differentiation of T & B cells  Interleukin-1 (IL-1)  Inflammatory response (fever)  Macrophage secretes IL-1 after presenting antigen  IL-1 + presented antigen  Activates helper T Cell to produce IL-2  Interleukin-2 (IL-2) “The Recruiter”  Helps B cells that have contacted antigen to differentiate into antibody-secreting plasma cells  Helps Cytotoxic T cells become active killers

 Cytotoxic T cells (Killer T cells) produces killer substances  Active against viruses/bacteria within infected cells  Will be eliminated from the body  Depends on MHC Class I AND II

 Viral proteins associate with MHC I  Transported to surface  Recognized by cytotoxic T cell (Killer T Cell)  Binds to specific fragment shape of MHC I and foreign protein shape (FITS TO ONLY FOREIGN)  ACTIVATES AND PREPARES KILLING SUBSTANCES  Waits for permission from Helper T cell

 Helper T Cell  Activated by interaction of APC (Antigen Presenting Cell) presenting fragments via MCH II molecules Just like in Humoral Response!  Helper T Cell releases cytokines (IL-2)  SIGNALS KILLER T CELL TO RELEASE THE KILLING SUBSTANCES!  Kills cell that contains virus Eventually rid body

 MHC Class I / II expressed on surface of tissue  Different shapes than our own  Recognized by helper and killer T cells See it as out own MHC molecules “presenting” foreign fragments Perceived as our own tissue that is “infected” REJECTION-IMMUNE SYSTEM ATTACKS!  10/12 MHCs must match  Immunosuppressant medication