1. Natural resistance to infection Immunity the condition of being immune; the protection against infectious disease conferred either by the immune response generated by immunization or previous infection or by other nonimmunologic factors. Types of Acquired Immunity Acquired immunity refers to the protection an animal developed against certain types of microbes or foreign substance. Acquired immunity is developed during an individual s lifetime.

2. Immuntmity can be acquired either actively or passively. : Immunity is acquired actively when a person is exposed to microorganisms or foreign substances and the immune system responds.

4. Naturally acquired Immunity Acquired Immunity Naturally acquired active immunity is obtained when a person is exposed to antigens in the course of daily life. Once acquired, immunity is lifelong for some diseases, such as measles. For other diseases, especially infectious diseases, the immunity may last for only a few years. Naturally acquired passive immunity involves the transfer of antibodies from a mother to her infant. A pregnant woman’s body passes some of her antibodies to her fetus across the placenta, a mechanism known as transpiacental transfer. If the mother is immune to Diphtheira, rubella, or polio, the newborn will be temporarily immune to these diseases as well. Certain antibody also passed from the mother to her nursing infant in breast milk, especially in the first secretions coostrum.

5. In the infant, immunity generally lasts _only as long as the transmitted antibodies are present— a few weeks or months. These maternal antibodies are essential for providing immunity to the infant until its own immune system matures. Colostrum is even more important to some other mammals, Calves, for example, do not have antibodies that cross the placenta and rely on colostrum ingested during the first day of life for this type of immunity. Researchers often specify fetal calf serum for certain experimental or clinical uses because it does not contain maternal antibodies.

6. Artificially Acquired Immunity Artificially acquired active immunity results from vaccination. Vaccination, also called immunization, introduces specially prepared antigens called vaccines into the body. Vaccines may be inactivated bacterial toxins (toxoids), killed microorganisms, living but attenuated (weakened) microorganisms, or parts of microorganisms such as capsules. These substances can no longer cause disease, but they can still stimulate an immune response, much as naturally acquired pathogens do.

7. Artificially acquired passive immunity Artificially acquired passive immunity involves the introduction of antibodies (rather than antigens) into the body. These antibodies come from an animal or person who is already immune to the disease. The antibodies are found in the serum of the immune animal or individual. Because most of the antibodies remain in the serum, antiserum has become a generic term for blood-derived fluids containing antibodies. Hence, the study of reactions between antibodies and antigens is called serology.

8. Mechanism of immune response to antigen and cell mediated immunity

9. Cellular Components of Immunity Learning Objective T cells are the key cellular component of immunity. Like B cells and all other cells involved in the immune response, T cells develop from stem cells in the bone marrow. T cells are influenced by the thvmus gland, where they differentiate into mature cells next they migrate to lymphoid organs, where they are apt to encounter antigens. Like the B cells of humoral immunity,T cells use anti-receptors to recognize and react to an antigen.

10. like B cells, each T cell can react specifically with only a single antigen. As in humoral immunity, clonal selection is the mechanism by which T cells with particular antigen receptors are stimulated to be differentiated into the effector T cells that carry out cell—mediated immunity and proliferate. As with B cells, some of the effector (antigen stimulated) T cells become memory cells, which provide the basis for a secondary immune response if they encounter the same antigen later. The body’s ability to make new T cells decreases with age, beginning in late adolescence. Eventually, the T-cell-producing thymus gland becomes inactive, and bone marrow produces fewer B cells. However, sufficient T and B memory cells survive to make immunization effective for such diseases as influenza and pneumococcal pneumonia.

11. Types of TCells There seem to be four main functional types of T cells: helper T (T H ) cells, cytotoxic T (Tc) cells, delayed hypersensitivity T (T D ) cells, and perhaps, suppressorT (Ts) cells. Mature T cells of these four types can be identified by their characteristic cell—surface molecules. Another classification of T cells is based on the type of cell-surface receptor called CD, for clusters of differentiation. Two types of these receptors are CD4 and CD8. CD4 cells are primarily helper T cells, whereas CD8 cells include both cytotoxic and suppressor T cells.n. These CD4 cells are vital to a vigorous immune response. Individual T cells interact specifically with only a single antigen. which must be displayed on a cell surface. These cells are known as antigen-presenting cells (APCs). The primary APCs are macrophages or dendritic cells..The APC ingests and processes the antigen: it then displays fragments of the antigen on the cell surface. A T cell will recognize an antigenic fragment on an APC only if it is in close association with certain cell—surface self molecules. These self molecules are components of the major histocompatibility complex (MHC). The particular MHC proteins a person carries

12. In helper T cell activation, 1)an antigen presenting cell first encounters and processes antigen 2) Binding of antigen to T H cell to an antigen-MHC corn the APC stimulates the APC to secrete the cytokine IL01 3) The IL-1 in turn activates theTH cell, which begins to synthesize interleukin—2 (IL—2).The IL—2 secreted by TH cell returns to receptors on the surface of the cell, which then begins to proliferate and differentiate into mature TH cells. Only TH cells that have been stimulated by an antigen have receptors for IL-2, so although the cytokine IL-2 is nonspecific, the resulting T H cells are specific to that antigen. 4) These activated TH cells then begin to produce IL-2 and other cytokines. This IL-2 in turn stimulates other TH cells specific to that antigen to proliferate and mature.

13. Cytotoxic T Cells Cytotoxic T (Tc) cells destroy target cells on contact. Because viruses, and some bacteria, reproduce within host cells, they cannot be attacked there by antibodies. The T cell attacks instead—for example, the virus- infected host cell. T cell binds with the MHC- antigen complex on the cell’s surface and then releases a protein called perforin. The perforin forms a pore in the membrane of the target cell, causing lysis.Tc cells continue their activity as long as the stimulating antigen persists; when it disappears, they undergo apoptosis. Cytotoxic T cells recognize and lyse cancerous cells.)