Innate Immunity Cells, Hematopoiesis, and Key Mechanisms
Characteristics of Innate Immunity
Innate Immunity: Rapid response (minutes to hours), non-specific (broad target recognition), with no immunological memory (reacts identically upon repeat exposure). Primary soluble factors include cytokines, chemokines, complement proteins, and acute-phase proteins.
Hematopoietic Lineage of Innate Cells
- Pluripotent Hematopoietic Stem Cell (Bone Marrow)
- Myeloid Progenitor
- Neutrophils
- Eosinophils
- Basophils
- Monocytes
- Macrophages
- Dendritic Cells
- Lymphoid Progenitor
- Natural Killer (NK) Cells
- Myeloid Progenitor
Granulocytes and Mast Cells
- Neutrophils (PMNs): Account for most white blood cells (50–70%). They act as first responders, arriving within 6–12 hours via chemotaxis. They phagocytize pathogens, release Reactive Oxygen Species (ROS) and proteolytic enzymes, and form Neutrophil Extracellular Traps (NETs). Short-lived; they are the primary constituent of pus.
- Mast Cells: Tissue-resident granulocytes located in connective tissue and mucosa. Primary activators of acute inflammation. They degranulate pre-formed histamine, heparin, and chemotactic factors (NCF, ECF-A) upon stimulation; they also synthesize leukotrienes, prostaglandins, and cytokines (TNF-α, IL-4).
- Basophils: Circulating granulocytes (<1% of WBCs) that mirror mast cell function. Express high-affinity Fc receptors for IgE. Crucial for type I hypersensitivity and defense against parasitic helminths.
- Eosinophils: Make up 1–3% of WBCs. Specialize in attacking large, non-phagocytosable multicellular parasites (helminths) using basic crystal granules. Participate in late-phase allergic responses and asthma.
Mononuclear Phagocyte System (MPS)
- Monocytes: Immature phagocytes circulating in blood (lifespan ~2 days) before migrating into tissues to mature into macrophages or dendritic cells.
- Macrophages: Highly efficient, long-lived tissue-resident phagocytes. Arrive at inflammatory sites within 3–7 days. They clean up apoptotic debris, release pro-inflammatory cytokines (IL-1, IL-6, TNF-α), aid in wound repair, and act as professional Antigen-Presenting Cells (APCs).
- Fixed Macrophages: Kupffer cells (liver), Microglia (brain), and Alveolar macrophages (lungs).
- Dendritic Cells (DCs): Primary cellular bridge between innate and adaptive immunity. Abundant in peripheral tissues (skin, GI tract). Immature DCs actively phagocytize antigens; mature DCs migrate to secondary lymphoid organs to present antigens via MHC molecules to naive T cells. They also produce Type I Interferons (IFN-α/β).
Innate Lymphocytes
Natural Killer (NK) Cells: Cytotoxic lymphoid cells that do not express specific antigen receptors. They target virally infected cells and tumor cells lacking self Killer Inhibitory Receptors (KIR) or MHC Class I molecules. They kill targets by releasing perforin (forms membrane pores) and granzymes (induce apoptosis).
Key Immunological Terminology
- Antigen:
- Any foreign invader or substance that the body perceives as a threat.
- Antibodies (Immunoglobulins):
- Proteins produced by plasma cells that bind to antigens to agglutinate, opsonize, or neutralize them.
- Agglutination:
- The binding or clumping of antigens together by antibodies.
- Opsonins:
- Molecules (such as IgG and C3b) that tag microorganisms for phagocytosis and destruction.
- Neutralization:
- The process by which antibodies block receptor sites on pathogens, rendering them inactive.
- Anaphylatoxins:
- Complement fragments (C3a, C4a, C5a) that induce rapid mast cell degranulation and histamine release.
- PAMPs (Pathogen-Associated Molecular Patterns):
- Target molecules recognized by pattern recognition receptors found within or on the surface of macrophages, dendritic cells, and epithelial cells.
- DAMPs (Danger-Associated Molecular Patterns):
- Signals released by damaged, stressed, or dying host cells.
- Precipitation:
- The conversion of a soluble antigen into an insoluble antigen complex.