The Adaptive Immune system by Dr Samuel Munjita-unza

The Adaptive Immune system Features of adaptive immunity Made up of the humoral immunity and cell mediated immunity Response is slow(1-2 weeks) but this varies from one person to another It is very specific Not always present unless it is a second exposure to the same pathogen Has memory - upon re-exposure to a pathogen, memory cells mount a much faster and more potent second response Cells of the adaptive immune system Lymphocytes and Lymphocyte Subpopulations. Several subpopulations of lymphocytes have been defined: a. B lymphocytes : precursors of antibody-producing cells, known as plasma cells. b. T lymphocytes, or T-cells, which are further divided into several subpopulations: i. Helper T lymphocytes (TH) : play a very significant amplification role in the immune responses. Two functionally distinct subpopulations of T helper lymphocytes have been well defined : a. TH1 lymphocytes: assist the differentiation of cytotoxic cells and also activate macrophages. Activated macrophages play a role as effectors of the immune response. b. TH2 lymphocytes: involved in the amplification of B lymphocyte responses. These amplifying effects of helper T lymphocytes are mediated in part by soluble mediators interleukins—and in part by signals delivered as a consequence of cell-cell contact. ii. Cytotoxic T lymphocytes : which are the main immunological effector mechanisms involved in the elimination of non-self or infected cells. NB: T cells are more numerous in paracortical areas of lymph nodes. B cells are more numerous in germinal centers of lymph nodes Lymphocytes and Lymphocyte Subpopulations Contd’ Lymphocytes: -Originate from hematopoietic stem cells in red bone marrow. Immature lymphocytes are identical when released from bone marrow -Determination of which lymphocyte (B or T) depends on where in the body it becomes immunocompetent (able to recognize a specific antigen by binding to it) -T cells formed in the thymus (2-3 days) where the T cells are selected for their ability to identify foreign antigens. There are two types of selections: a. Negative selection – lymphocytes that strongly attack self-antigens are destroyed in the thymic medulla b. Positive selection – weakly anti-self lymphocytes continue to develop and the ones that can best recognize self when attached to antigens are identified. This occurs in the thymic cortex. Cells of the adaptive immune system Contd’ 2. Antigen-presenting cells(APCs) Major types are: i. Dendritic cells (interstitial cells of connective tissues and Langerhans’ cells of the skin epidermis), ii. Macrophages, and activated B lymphocytes Engulf particles and presents fragments of these antigens on their own surfaces where they can be recognized by T cells APCs secrete proteins that activate T cells. In turn, activated T cells secrete chemicals (cytokines) that activate macrophages and increase DC maturation. APCs and lymphocytes are found throughout the lymphatic system. DCs are more numerous in germinal centers of lymph nodes Other cells that play a role in adaptive immune system Phagocytic and cytotoxic cells -Such as monocytes, macrophages, and granulocytes, also play significant roles as effectors of the immune response. -Once antibody has been secreted by plasma cells and is bound by the microbes, cells, or compounds that triggered the immune response, it is able to induce their ingestion by phagocytic cells. If bound to live cells, antibody may induce the attachment of cytotoxic T cells and NK cells that cause the death of the antibody-coated cell (antibody-dependent cellular cytotoxicity; ADCC). -The ingestion of microorganisms or particles coated with antibody is enhanced when an amplification effector system known as complement is activated. Natural killer (NK) cells -play a dual role in the elimination of infected and malignant cells. -They can participate in the elimination of antibody-coated cells by ADCC. Branches of adaptive immunity Humoral immunity or antibody-mediated immunity -consists of antibodies circulating in the fluids of the body - The antibodies are produced by B lymphocytes (B-cells) - Works best against bacteria, toxins, and virus that are free in body fluids Cellular or cell-mediated immunity -Involves T cells that target infected tissue cells directly by lysing foreign cells (cytotoxicity) or indirectly by releasing chemicals (cytokines) that enhance the inflammatory response or activate other lymphocytes or macrophages. -Works best on bacteria- or virus-infected cells, fungi, protozoa, tissue grafts and cancer cells Humoral immunity or antibody-mediated immunity B cells produce antibodies = humoral /antibody-mediated …