Acquired Immunity Defends Against Infection of Body Cells

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 CD 8, 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 o o Ig. M Ig. G Ig. A Ig. E Ig. D

Ig. M • 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.

Ig. G • 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 Ig. M

Ig. A • 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.

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

Ig. D • Ig. D 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.