Ritu's Biology Hub

Continued from Part 1

• Granulocytes

  – Have granules in cytoplasm containing reactive substances. 3 types – Basophils, Eosinophils, Neutrophils.

  • Basophils – Irregular nucleus – 2 lobes, cytoplasmic granules that stain bluish-black. Non-phagocytic cells. Release special substances from their granules under specific stimulations – Histamine, postaglandin, serotonin, leukotrienes – vasoactive mediators as they influence the tone and diameter of blood vessels. High affinity receptors for IgE. Once coated with IgE Abs, binding of Ags can trigger the release of vasoactive mediators. Associated with allergic responses.
  • Eosinophils – Bilobed nucleus connected by a slender thread of chromatin, have cytoplasmic granules that stain red with acidic dyes. Defensive against protozoans and helminthic parasites by releasing cationic peptides and reactive O₂ intermediates in the extracellular fluid. Migrate from the bloodstream to tissue spaces especially mucus membrane. They damage parasite plasma membrane and their count increases in allergic response by releasing histamine or aryl-sulphatase from their granules.
  • Neutrophils – Also called Polymorphonuclear Leukocytes (PMNLs). Multilobed, stain at neutral pH. Have inconspicuous organelles – 1^o and 2^o
    • 1^o – Peroxidase, Lysozyme, Defensive, other hydrolytic enzymes.
    • 2^o – Lysozyme, Lactoferrin, Collagenase, Cathelicidins

    Granules – Accomplish intracellular digestion of ingested microbes. They also generate additional antimicrobial substances by using O₂-dependent and independent mechanisms to kill microorganisms. Like macrophages, have receptors for Abs and complement system and unlike them do not reside in healthy tissue but instead go and accumulate at site of injury and become the principle phagocytic cell.

• Mast Cells – Bone marrow derived cells that are involved in allergic responses just as basophils secreting histamine, etc.

• Dendritic Cells – Constitute 0.2% WBCs. Present in skin and mucus membrane of nose, lungs and intestines. They phagocytose and present Ags – Ag presentation. Dendritic cells recognize PAMPs on the pathogens. These molecular patterns enable the dendritic cells to differentiate between self and non-self cells. Using the PRRs (Pathogen recognition Receptors) on them they bind to pathogens and phagocytose them. The dendritic cells then migrate to lymphoid tissue as activated cells that trigger the activation of T-cells by presenting Ags. Activation of T-cells is required for an effective specific immune response

• Natural Killer Cells (NKCs) – Large, granular lymphocytes of the Innate immunity system. Destroy malignant and cells infested with microorganisms. Kill by 2 ways –
  • ADCC – i.e. Ab-dependent cell-mediated cytotoxicity; Recognize target cells coated with Abs and thereafter destroy them
  • All nucleated host cells have a specialized protein receptor – Class I MHC Ag. When these cells lose the receptors, NKCs recognize these virally infected or cancerous cells and destroy them by forming pore-forming proteins and enzymes – granzymes that together lyze and kill them

Immune Mechanism

All nucleated cells of the body have a glycoprotein – MHC class I proteins on the surface including macrophages and dendritic cells. However, these cells also have MHC class II proteins. Cells having MHC Class I proteins process self proteins and display them on the surface via these surface proteins. On the other hand, cells having Class II proteins process non-self or foreign proteins. Virus-infected cells process proteins via Class I and not Class II proteins. However, when phagocytic cells engulf these cells, they process the non-self proteins via Class II proteins. The displayed proteins are recognized by specialized T-cells. MHC Class I proteins are recognized by T_C cells (Cytotoxic T-Cells) which kill the virus-infected cells when the virus has gained entry on its own. T_H cells (Helper T Cells) recognize MHC Class II proteins on APCs (Antigen Presenting Cells).

Points to Remember

• Important phagocytes – a. Neutrophils, b. Macrophages, c. Dendritic Cells
• Macrophages and Dendritic Cells are restricted within tissues, Ag presenting cells and process foreign proteins.

Continued to Part 3