The Immune System

The Immune System

• Primary function: to find and kill invasive microbes, including bacteria, fungi, viruses, and parasites.
• Body's first line of defense: physical barriers (skin, acidity of the stomach lining, flow of urine, low pH of the female reproductive tract) that keep pathogenic microbes from accessing human tissue.

Differentiation between Self and Non-Self

• The immune system uses proteins and carbohydrates on cell surfaces as antigens to differentiate between self and non-self.
• Antigens are substances that initiate an immune system response, often found on the surfaces of bacteria and viruses.

Innate and Adaptive Immunity

• Innate responses are not directed against a particular invader and work in the same way, regardless of the invader.
• Adaptive responses are directed against particular pathogens and foreign invaders, relying on immune memory, which gives rise to long-term resistance or protective immunity against a specific foreign substance or invader.

Tissues of the Immune System

• Central Lymphoid Tissues:
  • Bone Marrow: differentiation of hematopoietic stem cells into all cell types except mature T cells, and maturation and selection of B cell progenitors.
  • Thymus: maturation and selection of precursor T cells, resulting in naïve T cells that recognize foreign antigen and can distinguish between self and non-self.
• Peripheral Lymphoid Tissues:
  • Lymph nodes: located where lymph channels converge, cells and antigen flow slowly through, allowing T cells, B cells, macrophages, and dendritic cells to interact.
  • Spleen: filters antigen from the blood, and WBC interact in white pulp similar to lymph nodes.
  • Lymph nodules: mucosa-associated lymphoid tissues (MALT) that include tonsils, appendix, and Peyer's patches, with specialized phagocytic epithelial cells that transfer antigen into the lymphoid tissue.

Cells of the Immune System

• Derived from hematopoietic stem cells in the bone marrow.
• Hematopoietic stem cells differentiate into myeloid and lymphoid lineages.
• Myeloid lineage cells include granular phagocytic leukocytes (monocytes/macrophages, granulocytes, and dendritic cells) and all other non-lymphoid blood cells.
• Lymphoid cells include small lymphocytes (B and T cells) and the larger granular natural killer cells (NK cells).

Leukocytes

• Monocytes/Macrophages:
  • Monocytes circulate in blood for ~1 day and are the largest leukocyte.
  • Monocytes mature into macrophages when they migrate into tissues, acquiring greater phagocytic ability.
  • Macrophages engulf and kill invading microorganisms and infected cells, clean up dead cells and cellular debris, and help induce/maintain inflammation by secreting signaling proteins (cytokines) to activate and recruit other immune cells.
• Neutrophils:
  • Primary role: phagocytosis of bacterial invaders.
  • Most abundant leukocytes (50-70% of WBC).
  • First to arrive at the site of acute inflammation, can form pseudopods and are highly mobile.
  • Engulf and destroy bacterial invaders using degradative enzymes (lysozymes) and antimicrobial substances in their cytoplasmic granules.
• Eosinophils:
  • 1-3% of WBC.
  • Kill parasitic worms.
  • Recruited to sites of acute inflammation.
  • Longer life span than neutrophils.
  • Involved in chronic inflammation.
  • Granules contain digestive enzymes, lysozyme, peroxidase, and inflammatory chemicals.
  • Key component of allergic asthma and other allergic reactions.
• Basophils:
  • Involved in allergic reactions.
  • Granules contain histamine, which causes blood vessels to dilate and smooth muscle to contract.

Learn about the primary function of the immune system, its first line of defense, and how it differentiates between self and non-self. Understand the role of proteins and carbohydrates on cell surfaces as antigens.