Host response of infection

• Antigen

  A substance recognised by the immune system, whether by the B cell or the T cell, that serves as the target of the immune response but may not necessarily lead to an immune response
• Epitope

  The structure in the antigen that is recognized by the B cell or the T cell
• Hapten

  A substance that is of low molecular weight that can only induce an immune response if bound to another substance that is already immunogenic (carrier molecule)
• Immunogen

  Any substance capable of inducing an immune response, whether humoral or cell-mediated or both
• Immunology

  The study of the immune system and the immune response
• Properties that make an antigen immunogenic

  • Foreignness and genetic composition
  • Chemical composition and complexity
  • Molecular size and stability
  • Mode of entry of the antigen
• Antigens are genetically foreign to the host or recognized by the body as non-self
• Examples of genetic foreignness

  • If a pig's heart is transplanted into a human being, the human body will likely develop an immune reaction leading to rejection
  • People may have different reactions to the same environmental component (e.g. dust) due to genetic differences
• Most organic substances can be antigenic except for pure lipids and nucleic acids
• Proteins

  The most immunogenic chemical grouping because they are larger molecules with more complex structures
• Glycoproteins

  More antigenic than pure proteins because their structure is more complex
• Molecular weight

  Molecules with molecular weights below 10,000 daltons are weakly immunogenic or not immunogenic at all, those with molecular weights greater than 10,000 daltons are very potent immunogens
• Some high molecular weight substances can break up into smaller molecules once they enter the body, losing their immunogenicity
• Dose and mode of antigen administration

  Can affect immunogenicity - smaller doses may be needed for protein antigens compared to polysaccharide antigens, and some antigens may not elicit a reaction intramuscularly but may provoke a good response when given subcutaneously
• Central (primary) lymphoid organs

  • Bone marrow
  • Thymus
• Peripheral (secondary) lymphoid organs

  • Lymph nodes
  • Spleen
  • Mucosa-associated lymphoid tissues (MALT) including tonsils, adenoids, Peyer's patches, appendix
• Types of white blood cells

  • Granulocytes (e.g. neutrophils)
  • Lymphocytes
  • Monocytes and macrophages
• Neutrophils

  Play a major role in acute inflammation and bacterial infections
• Lymphocytes and macrophages

  Mainly involved in chronic inflammation
• Lymphocytes

  The predominant inflammatory cells in viral infections
• Macrophages

  Predominant in chronic inflammation
• Antigen-presenting cells

  • Macrophages
  • B cells
  • Dendritic cells
  • Langerhans cells
  • Kupffer cells
  • Glial cells
• Professional antigen-presenting cells

  Macrophages, B cells, dendritic cells
• Follicular dendritic cells

  Antigen-presenting cells found in association with lymphoid follicles in lymph nodes
• Interdigitating dendritic cells

  Langerhans cells in the skin that bring antigens to the paracortical zone of the lymph node
• Dendritic cells are considered the true link between innate and adaptive immunity
• Other white blood cells of the innate immune system

  • Eosinophils
  • Basophils
  • Platelets
• Eosinophils


  Possess eosinophilic granules that play a role in type I hypersensitivity reaction or allergy, and secrete major basic protein that is toxic to parasites
• Basophils


  Play a role in allergies, their granules contain histamine which is responsible for changes in the initial phase of an allergic reaction
• Platelets


  Membrane-bound cell fragments derived from megakaryocytes, mainly involved in blood coagulation but also secrete substances that play a role in inflammation
• Natural killer (NK) cells

  Large granular lymphocytes that are part of the innate immune system, originally classified as cytotoxic T cells but now recognized as distinct due to lack of T-cell receptor
• Cells of adaptive immunity

  • T lymphocytes
  • B lymphocytes
• B cells


  Mature in the fetal liver and adult bone marrow, located in germinal centers of lymph nodes and white pulp of spleen, differentiate into antibody-producing plasma cells and memory B cells, also function as antigen-presenting cells
• T cells


  Located in paracortical and interfollicular areas of lymph nodes and white pulp of spleen, involved in cell-mediated immunity, further differentiate into CD4+ helper T cells, CD8+ cytotoxic T cells, and memory T cells
• Innate immunity


  Also known as natural immunity, active from birth, non-specific, includes physical/chemical barriers and processes like phagocytosis and inflammation, activated within minutes, does not improve or have memory
• Functions of innate immunity

  • Killing invading microorganisms
  • Activating adaptive immune responses
• First line of innate defense

  • Physical and chemical barriers like skin, fatty acids, sweat, normal flora
• Pathogen-associated molecular patterns (PAMP)

  Molecular patterns recognized by innate immune cells and soluble mediators
• Pattern recognition receptors


  Allow inflammatory cells to act on pathogenic organisms
• Membrane attack complex (MAC)


  Produced by activated complement system to help degrade antigens