Immune system - basic concepts of structure and functions. Cells and Organs of the Immune System. Differentiation of stem cells into immunocompetent T and B lymphocytes.
- The immune system is a complex biological system in multicellular organisms designed to resist and defend against pathogenic microorganisms.
- It recognizes and tolerates self (own body) and recognizes and rejects non-self (foreign agents).
- It aims to preserve genetic and antigenic integrity against foreign agents and altered self-cell antigens (e.g., in autoimmune diseases or tumours).
- The immune system is composed of lymphoid organs and immune cells that defend the body against pathogens and malignancies
- It is broadly divided into:
- Innate immunity (Natural Resistance): Rapid and non-specific defences . Important in the first hours and days of infection. Consists of physical, chemical, molecular, and cellular defences.
- Adaptive immunity (Acquired Immunity): Slower but specific and memory-based Provides long-lasting protection through immune memory. Specific reactions of the immune system. Crucial for recovery in severe infections.
Cells of the Immune System
Leukocytes, arising from pluripotent stem cells in the bone marrow, differentiate into two main lineages: myeloid and lymphoid.
- Myeloid lineage includes phagocytes and granulocytes:
- Macrophages: Mature form of monocytes, present in almost all tissues. Important first-line defence in innate immunity via phagocytosis. Also function as antigen-presenting cells (APCs) for the adaptive response.
- Neutrophils: Most abundant granulocyte, important cell in innate immunity. Perform phagocytosis and destroy microorganisms via antimicrobial substances in granules. Short-lived and motile.
- Dendritic cells: Differentiated monocytes. Function as key APCs. Capture pathogens and present antigens to T cells in lymph nodes, initiating adaptive responses
- Lymphoid lineage gives rise to the effector cells of the specific immune response:
- B lymphocytes (B cells): Mediate humoral immunity Only cells capable of producing antibodies. In response to antigens, they differentiate into antibody-secreting plasma cells. Can also function as APCs.
- T lymphocytes (T cells): Mediate cell-mediated immunity (CMI ]. Antigen receptors recognize MHC molecules on the surface of APCs or infected cells. Subtypes include
- CD4+ T cells: Mature T helper (TH) cells. Recognize antigen in association with MHC Class II molecules. Help B cells produce antibodies and help phagocytic cells destroy ingested pathogens. Secrete cytokines that activate and regulate other immune cells.
- CD8+ T cells: Mature cytotoxic T (TC) cells. Recognize antigen in association with MHC Class I molecules. Kill or lyse intracellular microbes or infected cells.