Acquired Immunity Defends Against Infection of Body Cells and Fluids By: Jonah Harrington, Josh Yi

Introduction Acquired immunity is based on both a humoral immune response a cell-mediated immune response. Humoral immune response involves the activation and clonal selection of effector B cells, which secrete antibodies that circulate in the blood and lymph.

Critical Parts Helper T cells Cytotoxic T Cells Antibody Classes The Role of Antibodies in Immunity Active and Passive Immunization Immune Rejection Blood groups Tissue and Organ Transplant

Helper T Cells Helper T Cells are a type of T cells that play in important role in the immune system like helping the activity of other immune cells by releasing T cells cytokines They are activated by encounters with antigen- presenting cells, helper T cells play a central role in enhancing humoral and cell-medicated responses. Three Principal types of antigen-presenting cell and interact with helper T cells in different contexts: o Dendritic cells Important in triggering a primary immune response. o Macrophages Initiates a secondary immune response by presenting antigens to memory helper T cells. o B Cells Humoral response relies mainly on B cells to present antigens to help T cells.

Cytotoxic T Cells Cytotoxic T cells are the effector cells in a cell-mediated immune response. They require signaling molecules from helper T cells as well as interactions with antigen-presenting cells. When they are activated, they eliminate cancerous body cells and body cells infected by viruses or other intracellular pathogens. A surface protein called CD8, found on most cytotoxic T cells, enhances the interaction between a target cell and a cytotoxic T cells. They are best known as the “killer cell” because they kill cancer cells, cells that are infected or damaged cells.

Cytotoxic T Cells (Cont’d) B Cells produce antibodies.

Antibody Classes The antibodies produced differ from the B cell receptor only in the constant region of the heavy chain. 5 Major types of heavy chain C regions determine the five major classes of antibodies o IgM o IgG o IgA o IgE o IgD

IgM They are the first Ig class produced after initial exposure to antigen; then its concentration in the blood declines. Its function is to promote neutralization and crosslinking of antigens; they are very effective in complement system activation.

IgG They are the most abundant Ig class in blood; also present in tissue fluids Their function is to promote opsonization, neutralization, and crosslinking of antigens; less effective in activation of complement system than IgM

IgA They present themselves in secretions such as tears, saliva, mucus, and breast milk. Their function is that they provided localized defense of mucous membranes by cross linking and neutralization of antigens. Their presence in breast milk confers passive immunity on nursing infant.

IgE IgE are present in blood at concentrations. They triggers release from mast cells and basophils of histamine and other chemicals that cause allergic reactions.

IgD IgD are present primarily on surface of B cells that have not been exposed to antigens They act as antigens receptor in the antigen stimulated proliferation and differentiation of B cells.

The Role of Antibody in Immunity The binding of antibodies to antigens can interfere with pathogen function in many ways. Neutralization: The binding to surface proteins of virus or bacterium, thereby blocking the pathogen’s ability to infect a host cell. Opsonization: Antibodies abound to antigens present a readily recognized structure for macrophages and therefore increase phagocytosis.

Active and Passive Immunization. Active Immunity: A type of immunity or resistance developed in an organism by its own production of antibodies in response Passive immunity: A type of immunity acquired by the transfer of antibody from one individual to another Vaccination/Immunization: introduction of antigens into the body causing active immunity to develop

Immune Rejections Like pathogens, cells from another person can be recognized and attacked by immune defenses. Example: Skin transplanted from one person to a genetically non identical person will look healthy for a week or so but then will be destroyed by the recipient's immune response. Keep in mind that the body’s hostile reaction to a transplant of other tissues or whole organs or to an incompatible blood transfusion is the expected reaction of a healthy immune system exposed to foreign antigen.

Blood Groups To avoid harmful immune reactions in human blood transfusions, there are different blood groups (types Red cells are designated as type A if they have A antigen molecules on their surface. Similarly, the B antigen is found on type B red blood cells; both A and B antigens are found on type AB red bloods; and neither antigen is fond on type 0 red blood cells. Transfusions with different bloods can cause chills, fever, hock, and kidney malfunction.

Tissue and Organ Transplants In the case of tissue of organ transplants or grafts, it is MHS molecules that stimulate the immune response that leads to rejection. To minimize rejections, physicians try to use donor tissue bearing MHC molecules that match those of the recipients as closely as possible. In addition, the recipient takes medicine that suppress the immune responses. However these medicine can leave the recipient more susceptible to infections during the course of the transplant and treatment.