Immunology: Antibody Functions and Mechanisms

Hemagglutination

• Red blood cell agglutination used to determine ABO blood types and detect influenza and measles viruses.

Opsonization

• Antibodies coat antigens, enhancing their ingestion and lysis by phagocytic cells.

Neutralization

• IgG inactivates viruses by binding to their surface, and toxins are neutralized by blocking their active sites.

Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)

• Destroys large organisms like worms.
• The target organism is coated with antibodies and bombarded with chemicals from non-specific immune cells.

Inflammation

• Essential for fighting infections and healing tissues.
• Can contribute to chronic inflammatory conditions.

Activation of Complement

• IgG and IgM trigger the complement system, resulting in cell lysis and inflammation.

Antibody Structure/Immunoglobulins

• Gerald Edelman and Rodney Porter:
  • Worked with IgG.
  • Edelman used analytical centrifugation.
  • Porter used the enzyme papain.
• Basic Structural Unit:
  • Four polypeptide chains (2 heavy chains, 2 light chains) linked by disulfide bonds.
• Key Fact: One virus or microbe can have several antigenic determinant sites or epitopes to which different antibodies may bind.
• Light Chains:
  • Small chains common to all immunoglobulins.
  • Molecular Weight: 25,000 Daltons.
  • 200-220 amino acids.
  • Two subtypes: kappa and lambda.
  • Kappa and Lambda have different amino acid sequences and are antigenically different.
  • About 65% of immunoglobulin molecules have Kappa chains, whereas 35% have Lambda chains.
  • One antibody has only one type of light chain, never both.
• Paratope:
  • Region on the antibody reacting to the epitope of an antigen.
• Clonal Selection Process:
  • Individual lymphocytes are genetically pre-programmed to produce one type of immunoglobulin.
  • A specific antigen finds or selects those particular cells capable of responding to it, causing them to proliferate.
• B Cell Production of Antibodies:
  • B cells develop from stem cells in the bone marrow.
  • After maturation, they migrate to lymphoid organs (lymph nodes, spleen).
  • When a B cell encounters an antigen, it is stimulated and divides into plasma cells, which secrete antibodies.

Properties of Antibodies

• Protein in nature
• High molecular weight
• Found in serum/plasma, CSF, saliva, seminal fluid.
• Possess high order of specificity

Agglutination

• Antibodies cause antigens to clump together.
• IgM is more effective than IgG in agglutination.

Svedberg Unit

• Indicates the sedimentation rate in an analytical ultracentrifuge.

Immunoglobulin Classes

• Sedimentation Coefficient | Molecular Weight
  • IgG | 7S, 150,000
  • IgA | 7S, 160,000 (IgA1), 350,000 (IgA2)
  • IgM | 19S, 900,000
  • IgD | 7S, 180,000
  • IgE | 8S, 190,000

Antibody Classification

• According to Temperature
  • Cold Antibodies: Reacts at 4°C (room temperature).
  • Warm Antibodies: Reacts at 37°C (Rh antibodies).
• According to Occurrence
  • Natural Antibodies: Appear without any apparent stimulus (e.g., anti-A and anti-B antibodies).
  • Immune Antibodies: Appear following the introduction of an antigen.
• According to Species That Produce Them
  • Isoantibodies: Antibodies produced after the introduction of the antigen from the same species.
  • Heterophile Antibodies: Antibodies produced after the introduction of the antigen from another species.
• According to Reaction with an Antigen
  • Immobilization of Motile Organisms
  • Agglutinins: Antibodies responsible for the immobilization of motile organisms and cell clumping.
  • Precipitins: Antibodies that cause precipitate formation by forming complexes with antigen molecules in solution.
  • Agglutinoids: Agglutinins modified by heat, losing their ability to agglutinate but still capable of combining with specific agglutinogens.
  • Hemagglutinins: Cause agglutination of RBCs.
  • Lysins: Cause dissolution of antigenic cells.
    • Bacteriolysin: Acts upon bacterial cells.
    • Hemolysin: Acts upon RBCs.
    • Leucocidin: Kills leukocytes with or without lysis.
• According to In-Vitro Behavior
  • Opsonin: Acts on bacterial invaders, weakening their resistance to phagocytosis by leukocytes.
• Point of Differentiation | Complete Antibody | Incomplete Antibody
  • Complete Antibody: Can agglutinate cells directly.
  • Incomplete Antibody: Cannot agglutinate cells directly, but can bind to antigens.

Emphysema

• Emphysema is a lung disease characterized by the breakdown of the alveoli (tiny air sacs in the lungs).
• It is caused by the release of elastase by white blood cells (WBCs) which breaks down the alveolar tissue.
• Alpha-1 antitrypsin (AAT) is a protein that normally inhibits elastase, preventing the destruction of the alveoli.
• A deficiency in AAT can lead to premature emphysema.

Alpha-1 Antitrypsin (AAT)

• AAT is a protease inhibitor that specifically targets elastase, an enzyme released from leukocytes during bacterial activity.
• AAT deficiency can lead to premature emphysema.
• In AAT deficiency, WBCs are not inhibited from releasing elastase, which can lead to the breakdown of the alveoli.

Haptoglobin

• Haptoglobin is a protein that binds to free hemoglobin in the bloodstream.
• It prevents the loss of hemoglobin and its constituent iron into the urine.
• Haptoglobin is used to evaluate the degree of hemolytic transfusion reactions and hemolytic disease of the newborn.
• Low levels of haptoglobin may indicate hemolysis (breakdown of red blood cells).

Ceruloplasmin

• Ceruloplasmin is a copper-binding protein that has enzymatic activities.
• It imparts a blue color to plasma and is a marker for Wilson's disease.

Wilson's Disease

• Wilson's disease is an autosomal recessive inherited disorder in copper metabolism.
• It results in copper deposition in the cornea, skin, liver, and brain.
• Kayser-Fleischer rings are copper deposits in the cornea, a characteristic symptom of Wilson's disease.

Menkes Kinky Hair Syndrome

• Menkes kinky hair syndrome is a copper metabolism disorder.
• It is characterized by kinky hair, due to a deficiency in copper.

Complement C3

• Complement C3 is the most abundant complement protein in serum.
• It plays a crucial role in the immune system.
• Complement C3 is activated through three pathways:
  • Classical pathway: Activated by antigen-antibody complexes.
  • Alternative pathway: Activated by microbial surfaces and other foreign substances.
  • Mannose-binding lectin pathway: Activated by mannose-binding lectin, a protein that binds to carbohydrates on the surface of microorganisms.

Cytokines

• Cytokines are small soluble proteins that regulate the immune system.
• They are produced by cells of the immune system in response to stimuli, such as bacterial products or foreign antigens.
• Cytokines induce the movement of cells or their immunological function.
• A cytokine storm is a massive overproduction and dysregulation of cytokines, leading to shock, multi-organ failure, and even death.

Interleukins

• Interleukins are a family of cytokines that mediate communication between white blood cells (WBCs).
• They are numbered from IL1 to IL32.

Interleukin 1

• Interleukin 1 (IL-1) is a family of cytokines that consists of IL-1 alpha, IL-1 beta, and IL-1 RA (receptor antagonist).
• IL-1 alpha: Remains intracellular within monocytes and macrophages, released only after cellular death. It attracts inflammatory cells to the site of infection.
• IL-1 beta: Is cleaved intracellularly and then secreted by monocytes. It induces fever, activation of phagocytes, and production of acute-phase reactants.
• IL-1RA: Competitively binds to the IL-1 receptor, preventing IL-1 from exerting its inflammatory effects.

Interleukin 2

• Interleukin 2 (IL-2) is a T cell growth factor that stimulates the growth and division of T cells, allowing for rapid expansion of the immune response.

Interleukin 6

• Interleukin 6 (IL-6) stimulates the production of acute-phase proteins and promotes the differentiation of B cells into plasma cells.

Acute Phase Reactants

• Acute phase reactants are proteins produced by the liver in response to inflammation, infection, or injury.
• They are produced within 12-24 hours in response to increased cytokine levels.

C-Reactive Protein (CRP)

• CRP is a positive acute-phase reactant.
• It is a ring-shaped protein composed of five identical subunits.
• CRP functions in opsonization, complement activation, and inflammation.
• It is a marker for pneumococcal infection and is used to assess cardiovascular risk.

Serum Amyloid A

• Serum amyloid A is an apolipoprotein synthesized in the liver.
• It is associated with HDL (high-density lipoprotein) cholesterol and removes cholesterol from cholesterol-filled macrophages at the site of injury.

Emphysema

• Emphysema is a lung condition where the alveolar sacs, or respiratory tract, are digested by elastase, an enzyme released by white blood cells (WBCs).
• This digestion process leads to the destruction of lung tissue, making it difficult to breathe.
• Alpha-1 antitrypsin (AAT) is a protein that inhibits elastase, preventing it from destroying lung tissue.
• A deficiency in AAT can result in premature emphysema.

Fibrinogen

• Fibrinogen is the most abundant coagulation factor found in plasma.
• It's produced in the liver and prevents bleeding disorders.
• When converted to fibrin, it forms a mesh-like structure that helps to stop bleeding.

Haptoglobin

• Haptoglobin binds to free hemoglobin, preventing its loss into the urine.
• Hemoglobin is nephrotoxic (toxic to the kidneys) and causes a range of health problems if not controlled.
• Low levels of haptoglobin may indicate a severe hemolytic transfusion reaction or hemolytic disease of the newborn.

Ceruloplasmin

• Ceruloplasmin is a copper-binding protein with enzymatic activity.
• It imparts a blue color to the protein and serves as a marker for Wilson's disease, an autosomal recessive disorder affecting copper metabolism.
• Wilson's disease leads to copper deposition in the cornea, skin, liver, and brain.

Kayser-Fleischer Rings

• Copper deposits in the cornea of patients with Wilson's disease can form rings, known as Kayser-Fleischer rings. These rings are a characteristic symptom of the disease.

Menkes Kinky Hair Syndrome

• Menkes kinky hair syndrome is another copper metabolism disorder.
• It results in abnormally curled hair due to problems with copper metabolism.

Complement C3

• Complement C3 is the most abundant complement protein found in serum. Involved in the following pathways:
  • Classical Pathway: Activated by antigen-antibody complexes
  • Alternative Pathway: Activated by microbial surfaces and foreign substances.
  • Mannose-Binding Lectin Pathway: Activated by mannose-binding lectin, a protein that binds to carbohydrates on the surface of microorganisms.

7 Acute Phase Reactants

• Acute phase reactants are proteins produced by the liver in response to inflammation, infection, or injury.
• They are produced within 12-24 hours after an increase in cytokines, such as Interleukin 1 beta, Interleukin 6, and Tumor Necrosis Factor (TNF alpha), which are produced by monocytes and macrophages at the site of inflammation.

C-Reactive Protein (CRP)

• CRP is a positive acute phase reactant that is ring-shaped and composed of five identical subunits.
• It plays a role in opsonization, complement activation, and inflammation.
• CRP is a marker for pneumococcal infection, as it precipitates with C-substance derived from the polysaccharide of bacteria.
• It is an early marker for individuals at risk for coronary heart disease. It binds to atherosclerotic plaques.
• High CRP levels (greater than 2mg/dl) are associated with a significant risk for cardiovascular disease.

Serum Amyloid A (SAA)

• SAA is an apolipoprotein synthesized in the liver.
• It is associated with HDL (high-density lipoprotein) cholesterol and helps to remove cholesterol from cholesterol-filled macrophages at the site of injury.

Alpha-1 Antitrypsin (AAT)

• AAT is a major component of the alpha-1 band of serum electrophoresis.
• It acts as a protease inhibitor, specifically for elastase.
• AAT prevents the constant release of digestive enzymes by WBCs after an infection has been neutralized.
• AAT deficiency can lead to premature emphysema.

Cytokines

• Cytokines are small soluble proteins that regulate the immune system.
• They are produced in response to various stimuli, such as bacterial lipopolysaccharides, flagella of some motile microorganisms, and other products of microorganisms.
• Cytokines regulate leukocyte activity and contribute to the elimination of infection.

Cytokine Storm

• A cytokine storm is a massive overproduction and dysregulation of cytokines, leading to shock, multiorgan failure, and even death.

Interleukins (ILs)

• Interleukins are soluble substances that allow communication between white blood cells (WBCs).
• They are numbered from IL1 to IL32.

Interleukin 1 (IL-1)

• IL-1 is a family that includes IL-1 alpha, IL-1 beta, and IL-1RA (receptor antagonist). - IL-1 alpha: Remains intracellular within monocytes and macrophages and can only be released after cellular death. - IL-1 beta: Cleaved intracellularly and then secreted by monocytes. It induces fever, activation of phagocytes, and production of acute phase reactants. - IL-1RA: Competitively binds to the IL-1 receptor, preventing IL-1 from exerting its inflammatory effects.

Interleukin 2 (IL-2)

• IL-2 is a T cell growth factor that stimulates the growth and division of T cells, allowing for a rapid expansion of the immune response.

Interleukin 6 (IL-6)

• IL-6 stimulates the production of acute-phase proteins, promotes the differentiation of B cells into plasma cells, and promotes the production of neutrophils.

Tumor Necrosis Factor (TNF)

• TNF is a cytotoxic agent that targets tumor cells and virally infected cells.
  • Tumor Necrosis Factor Alpha (TNF alpha): The most prominent member of the TNF family, with diverse biological functions.

Hot T-Bone Steak

• A mnemonic to remember the major cytokines:
  • IL-1
  • IL-2
  • IL-6
  • TNF

Explore the various roles of antibodies in the immune system, including hemagglutination, opsonization, neutralization, and more. This quiz covers key concepts in immunology related to antibody structure and functions, essential for understanding immune responses and their clinical implications.