Molecular Basis of ABO + Leucocytes

Leukocytes: Nucleated White Blood Cells

• Polymorphonuclear Cells (Granulocytes)
  • Neutrophils: First responders, phagocytize bacteria and debris, release enzymes for destruction.
  • Eosinophils: Combat parasites, involved in allergic reactions, modulate inflammatory responses.
  • Basophils: Release histamine and heparin, promote inflammation, involved in allergic reactions.
• Mononuclear Cells
  • Lymphocytes: T cells (cellular immunity), B cells (humoral immunity), NK cells (innate immunity).
  • Monocytes: Differentiate into macrophages, phagocytize pathogens and debris, present antigens.

Granulocytes: Degranulation

• Granulocytes, upon activation by chemical stimuli, undergo degranulation.
• Vesicle membranes fuse with the cell plasma membrane, releasing granule contents, such as enzymes and cytokines, to fight pathogens or modulate immune responses.

Neutrophil Function

• Fastest response of all WBCs to bacteria; crucial in acute infections.
• Direct actions against bacteria:
  • Release lysozymes: Destroy/digest bacteria by breaking down cell walls.
  • Release defensin proteins: Act like antibiotics, poke holes in bacterial cell walls, disrupting their integrity.
  • Release strong oxidants (bleach-like chemicals): Destroy bacteria through oxidation.

Eosinophil Function

• Leave capillaries to enter tissue fluid, targeting areas with inflammation or parasitic infections.
• Release histaminase: Slows down inflammation caused by basophils, maintaining immune balance.
• Attack parasitic worms by releasing toxic substances, causing their destruction.
• Phagocytize antibody-antigen complexes, clearing them from the body to resolve immune reactions.

Basophil Function

• Involved in inflammatory and allergy reactions, contributing to hypersensitivity responses.
• Leave capillaries and enter connective tissue as mast cells, where they release mediators.
• Release heparin, histamine, and serotonin:
  • Heighten the inflammatory response, increasing blood flow and immune cell recruitment.
• Account for hypersensitivity (allergic) reaction, leading to symptoms like itching, swelling, and anaphylaxis.

Lymphocyte Types

• T cells: Thymus-derived lymphocytes, including helper T cells, cytotoxic T cells, and regulatory T cells.
• B cells: Lymphocytes that mature in the bone marrow, differentiating into plasma cells to produce antibodies.
• NK cells: Natural killer cells, providing innate immunity against infected or cancerous cells.

Lymphocyte Functions

• T-Cells:
  • Attach to antigen-bearing cells → cellular immune response, directly killing infected cells.
  • Release toxins which kill target cells, eliminating pathogens and abnormal cells.
  • Release cytokines (e.g., interleukins) which cause a cellular response to antigens, coordinating immune responses.
• B-Cells:
  • Differentiate into plasma cells and produce antibodies, targeting specific antigens for destruction.
  • Humoral response (fluids), providing systemic immunity through circulating antibodies.
  • Produce up to 2000 antibodies per second, rapidly neutralizing pathogens.
  • Antibodies travel through fluids to destroy antigens, marking them for phagocytosis or complement activation.
• Shared Functions of T-Cells and B-Cells:
  • Respond to recognized antigens and create clones of themselves, amplifying the immune response.
  • Possess millions of varieties, allowing recognition of a wide range of antigens.

Monocyte Function

• Take longer to get to the site of infection but arrive in larger numbers, providing sustained immune response.
• Become wandering macrophages once they leave the capillaries, phagocytizing pathogens and cellular debris.
• Destroy microbes and clean up dead tissue following an infection, promoting tissue repair.

Chemotaxis

• Chemotaxis: Movement of an organism in response to a chemical stimulus, guiding immune cells to infection sites.
• Neutrophils migrate to areas of infection or tissue injury, following chemical signals released by damaged cells.
• Chemotaxis is the force of attraction that determines the direction in which neutrophils move, attributed to substances liberated at sites of tissue damage, such as cytokines and chemokines.

Key Processes in Immune Response

• Chemotaxis: Directs immune cells to sites of infection or injury.
• Phagocytosis: Engulfs and destroys pathogens and debris.
• Adherence: Immune cells bind to target cells or pathogens.
• Deformability: Allows immune cells to squeeze through tissues.
• Killing: Destruction of pathogens by various mechanisms.
• Circulation: Immune cells travel through the bloodstream.
• Apoptosis: Programmed cell death to remove infected or damaged cells.

Cell Adhesion

• Cell adhesion: Process by which cells interact and attach to neighboring cells through specialized molecules on the cell surface, facilitating immune cell interactions.
• Cell adhesion occurs through interactions between cell-adhesion molecules (CAMs), transmembrane proteins on the cell surface, mediating cell-cell and cell-matrix interactions.

Cell Adhesion Molecules (CAMs)

• CAMs are a subset of cell adhesion proteins on the cell surface involved in binding with other cells or the extracellular matrix (ECM) in cell adhesion, enabling immune cell trafficking and interactions.
• CAMs are classified into four major families:
  • Integrins: Mediate cell-matrix interactions and leukocyte adhesion.
  • Immunoglobulins (Ig) superfamily: Involved in cell-cell recognition and adhesion.
  • Cadherins: Mediate cell-cell adhesion in tissues.
  • Selectins: Facilitate leukocyte rolling and adhesion to endothelial cells.
• Each adhesion molecule has a different function and recognizes different ligands, allowing specific cell interactions.

Leukocyte Adhesion and Migration Steps

1. Tethering: Initial weak adhesion to endothelial cells.
2. Rolling: Leukocytes roll along the endothelium.
3. Activation: Triggering of integrin activation.
4. Adhesion: Firm adhesion to the endothelium.
5. Transmigration: Leukocytes squeeze through the endothelium into the tissue.

ABO Blood Group System

• Approximately 30 human blood group systems have been recognized; the best known are ABO, Rh (Rhesus), and MN systems, which are crucial in transfusion medicine.
• "Blood group" applies to a defined system of RBC antigens (blood group substances) controlled by a genetic locus with a variable number of alleles (e.g., A, B, and O in the ABO system), determining blood type.
• The ABO blood group system denotes the presence of one, both, or neither of the A and B antigens on erythrocytes, influencing transfusion compatibility.

ABO Blood Group Antigens and Antibodies

• The ABO blood group system involves two antigens (antigen A and antigen B) and two antibodies (antibody A and antibody B) found in human blood, determining blood type and compatibility.
• Antigens are present on red blood cells (RBCs), and antibodies are in the serum, mediating immune responses to incompatible blood types.

Blood Type Compatibility

• Type AB is the universal recipient, able to receive blood from any ABO type.
• Type O is the universal donor, able to donate blood to any ABO type.

History of the ABO System

• Karl Landsteiner: Discovered the ABO blood groups in 1900 by observing red cells of some individuals were clumped (agglutination) by the serum of others, revolutionizing transfusion medicine.
• Later, a fourth group AB was discovered by his students, completing the ABO blood group system.
• Landsteiner also discovered that a person's serum contained antibodies against an antigen absent from his own cells, explaining transfusion reactions.

ABO Substances

• ABO substances are complex oligosaccharides (in most cells and certain secretions), determining ABO blood type.
• On RBC membranes, oligosaccharides are present in glycospingolipids; in secretions, they are in glycoproteins, influencing blood group antigen expression.
• Glycosphingolipids: Subtype of glycolipids containing the amino alcohol sphingosine; sphingolipids with an attached carbohydrate, found in cell membranes.
• Glycoproteins: Proteins with carbohydrate groups attached to the polypeptide chain, involved in cell signaling and recognition.

Glycoprotein Function

• Hormones: Regulate various physiological processes.
• Antibodies: Recognize and neutralize foreign antigens.
• Enzymes (blood clotting cascade): Catalyze biochemical reactions in blood coagulation.
• Structural components of extracellular matrix: Provide support and organization to tissues.
• Lysosomal enzymes: Degrade cellular components in lysosomes.
• Secretory proteins: Released from cells to perform various functions.
• Receptors: Bind to specific molecules to initiate cellular responses.
• Glycoproteins are proteins to which oligosaccharides are covalently attached, playing diverse roles in cellular processes.

Glycoprotein Structures

• Glycolipid: Lipid with attached carbohydrate.
• O-linked glycoproteins: Glycans attached to serine or threonine residues.
• N-linked glycoproteins: Glycans attached to asparagine residues.

Branched Glycoprotein Structures

• Asparagine (N)-linked glycans:
  • Oligomannose: High mannose content.
  • Hybrid: Mixture of mannose and complex glycans.
• O-GalNAc (O)-glycans:
  • Core 1: Gal-β1-3GalNAc.
  • Core 2: GlcNAc-β1-6(Gal-β1-3)GalNAc.
  • Core 3: GlcNAc-β1-3GalNAc.
  • Core 4: GlcNAc-β1-6(GlcNAc-β1-3)GalNAc.

Glycoprotein Components

• N-acetylneuraminic acid (NANA): Sialic acid.
• Galactose (Gal): Monosaccharide.
• N-acetyl glucosamine (GlcNAc): Amino sugar.
• Mannose (Man): Monosaccharide.
• Fucose: Deoxy sugar.

Glycoprotein Synthesis

• The protein portion of glycoproteins is synthesized on the RER, where translation occurs.
• Carbohydrate chains are attached to the protein in the lumen of the ER and the Golgi complex, undergoing glycosylation.
• Initial sugars are added to a serine or a threonine residue in the protein, initiating O-linked glycosylation.
• The carbohydrate chain is extended by sequential addition of sugar residues to the nonreducing end, forming complex glycan structures.

Transport of Glycoproteins in the Golgi Apparatus

• Glycoproteins to be secreted remain free in the lumen and are released when the vesicle fuses with the cell membrane, undergoing exocytosis.
• Glycoproteins to become components of the cell membrane are integrated into the membrane of secretory vesicles that bud from the Golgi and fuse with the cell membrane, becoming integral membrane proteins.

Formation of Sugars for Glycolipid and Glycoprotein Synthesis

• Transferases produce oligosaccharide and polysaccharide side chains for glycolipids and attach sugar residues to glycoproteins, catalyzing glycosylation reactions.
• Transferases are specific for sugar moiety and for donating nucleotide (e.g., UDP, CMP, GDP), ensuring precise glycan assembly.
• Sugar-nucleotides are used for glycoprotein, proteoglycan, and glycolipid formation:
  • Examples: UDP-glucose, UDP-galactose, UDP-glucuronic acid, UDP-xylose, UDP-N-acetylglucosamine, UDP-N-acetylgalactosamine, CMP-N-acetylneuraminic acid, GDP-fucose, GDP-mannose.

UDP-Glucose Metabolism

• Interconversion of monosaccharides and their derivatives, facilitating glycan synthesis.
• Glucose \longrightarrow Glucose-6-phosphate \longrightarrow Glucose-1-phosphate
• Glucose-1-phosphate + UTP \longrightarrow UDP-glucose
• UDP-glucose can be converted to:
  • Glycogen: Storage form of glucose.
  • Proteoglycans: Components of extracellular matrix.
  • Glycoproteins: Proteins with attached glycans.
  • Glycolipids: Lipids with attached glycans.
  • UDP-glucuronate: Involved in detoxification.
  • UDP-galactose: Used in lactose synthesis.

Synthesis of O-linked Glycosides

1. Protein is synthesized in the RER and extruded into the lumen, where glycosylation occurs.
2. Glycosylation begins with the transfer of an N-acetylgalactosamine into the R group of a serine or threonine residue, initiating O-linked glycan synthesis.
3. Transfer of N-acetylgalactosamine occurs from UDP-acetylgalactosamine, catalyzed by glycosyltransferases.
4. Glycosyltransferases are responsible for the stepwise synthesis of oligosaccharides, adding sugars to the growing glycan chain.

Synthesis of N-linked Glycosides

1. Protein synthesis begins and the polypeptide chain is extruded into the endoplasmic reticulum (ER), where glycosylation occurs.
2. A branched oligosaccharide is synthesized bound to dolichol pyrophosphate, acting as a carrier for glycan transfer.
3. The oligosaccharide is transferred from dolichol to an asparagine residue of the growing polypeptide chain, initiating N-linked glycan synthesis.
4. Trimming of the carbohydrate chain begins as the protein moves through the ER, removing specific monosaccharides.
5. In the Golgi, further trimming and/or addition of monosaccharides occurs, creating complex and high-mannose glycoproteins, tailoring glycan structures to specific protein functions.

Glycan Locations

• O-glycans: S/T (Serine/Threonine), attached to hydroxyl groups.
• N-glycans: N (Asparagine), attached to amide groups.

I-Cell Disease

• Caused by a deficiency of the ability to phosphorylate mannose, leading to lysosomal dysfunction.
• Characterized by skeletal abnormalities, restricted joint movement, coarse facial features, and severe psychomotor impairment, resulting from impaired lysosomal enzyme targeting.
• Death usually occurs by age eight years, due to complications from lysosomal storage defects.

Mechanism for Transport of N-linked Glycoproteins to the Lysosomes

• Prelysosomal enzymes with phosphorylated mannose are transported from the ER to the CIS Golgi, where they are recognized by mannose 6-phosphate receptors.
• Mannose 6-P receptors in the TRANS Golgi bind to these enzymes, ensuring their proper targeting to lysosomes.
• Transport vesicles carry the enzymes to the lysosome, and the receptors are recycled, maintaining efficient enzyme trafficking.

Summary of the Causation of I-Cell Disease

• Mutations in the gene encoding GlcNAc phosphotransferase, disrupting mannose phosphorylation.
• Lack of normal transfer of GlcNAc 1-P to specific mannose residues of certain enzymes destined for lysosomes, preventing proper enzyme targeting.
• These enzymes consequently lack Man 6-P and are secreted from cells (e.g., into the plasma) rather than targeted to lysosomes, leading to lysosomal deficiencies.
• Lysosomes are thus deficient in certain hydrolases, do not function properly, and accumulate partly digested cellular material, manifesting as inclusion bodies, causing cellular dysfunction.

Glycolipids

• Glycolipids are molecules that contain both carbohydrate and lipid components, found in cell membranes.
• Glycolipids are derivatives of ceramides in which a long-chain fatty acid is attached to the amino alcohol sphingosine; they are also called glycosphingolipids, playing roles in cell recognition and signaling.

Glycolipid Function

• Intracellular communication: Mediate cell signaling pathways.
• Cell recognition factors (carbohydrate residues in these oligosaccharides are the antigens of the ABO blood group substances): Determine blood type and cell identity.
• Cell surface receptors: Bind to specific ligands to initiate cellular responses.
• Glycosphingolipids differ from sphingomyelin in that they do not contain phosphate, and the polar head function is provided by monosaccharide or oligosaccharide attached directly to the ceramide by an O-glycosidic bond, influencing membrane properties and interactions.

Types of Glycolipids

• Neutral Glycolipids:
  • Cerebrosides (Ceramide monosaccharides):
  • Glucocerebroside: Cer-Glc.
  • Galactocerebroside: Cer-Gal.
  • Globosides (Ceramide oligosaccharides).
• Acidic:
  • Gangliosides (Ceramide oligosaccharide + NANA).
  • Sulfatides (galactocerebroside with sulfated galacto).

Neutral Glycosphingolipids

• The simplest neutral glycosphingolipids are the cerebrosides, which are ceramide monosaccharides that contain either galactose or glucose, found in neural tissues.

Examples of Globosides

• Cerebroside (glucocerebroside): Cer-Glc.
• Globoside (lactosylceramide): Cer-Glc-Gal.
• Globoside (Forssman antigen): Cer-Glc-Gal-Gal-GalNac-GalNac.

Acidic Glycosphingolipids

• Negatively charged at physiologic pH, due to the presence of NANA or sulfate groups.
• Gangliosides are ceramide oligosaccharides and contain one or more molecules of NANA, found in nerve cell membranes.
• Accumulation of NANA-containing glycosphingolipids in cells causes different disorders, such as Tay-Sachs disease.
• Sulfatides or sulphoglycosphingolipids are cerebrosides that contain sulfated galactosyl residues, found in brain and kidneys.

Gangliosides

• GM1, GM2, GM3: Different types of gangliosides with varying carbohydrate structures.

Biosynthesis of Glycosphingolipids (GSLs) in Humans

• L-serine + palmitoyl-co-enzyme A -> Ceramide.
  • UDP-glucose -> UDP.
  • UDP-glucose: N-acylsphingosine D-glucosyltransferase.
• Glucosylceramide.
  • Lactosylceramide.
  • Ganglio-series.
  • Lacto(neo)-series.
  • Globo-series.

Synthesis of Glycospingolipids

• Ceramide + Phosphatidylcholine -> Diacyglycerol + Sphingomyelin.
• Ceramide + UDP-glucose -> Glucocerebroside + UDP.
• Galactocerebroside + PAPS -> Sulfatide.
• Glucocerebroside + Two or more UDP-sugars -> Globoside + UDP.
• Ganglioside + CMP-NANA -> Ganglioside + CMP.
• Synthesis occurs in ER & Golgi by specific Glycosyl transferases, adding sugars to ceramide.
• Sequential addition of glycosyl monomers to acceptor molecule (ceramide), forming diverse glycosphingolipids.
• Added sugar is activated (e.g. UDP-gucose), facilitating glycosylation.

Structure of the Blood Group Substances

• Type O: H substance, the precursor to A and B antigens.
• Type A: N-acetylgalactosamine, added to the H substance.
• Type B: Galactose, added to the H substance.

ABO Blood Group Summary

• A antigen: N-acetyl-galactosamine, present on type A erythrocytes.
• B antigen: Galactose, present on type B erythrocytes.
• H antigen: Fucose, the precursor to A and B antigens.

Blood Group Substances Synthesis

• The surface of human erythrocytes is covered by a complex mosaic of specific antigenic determinants, many of which are complex polysaccharides, determining ABO blood type.
• The H gene codes for a fuc