Antibody or Immunoglobulins: Antigen-Antibody Binding, 5 Types of Antibodies YouTube Lecture Handouts

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Antibody or Immunoglobulins:Antigen-Antibody Binding,5 Types of Antibodies - IgM, IgD, IgA, IgE, IgG

Title: Antibody or Immunoglobulins Antigen-Antibody Binding 5 Types of Antibodies

  • Antibodies are glycoproteins, termed as immunoglobulins (Igs) , which are produced in response to an immune reaction and specifically bind to antigens responsible for initiating the reaction.
  • Antibodies are specialized, Y-shaped proteins that bind like a lock-and-key to the body՚s foreign invaders whether they are viruses, bacteria, fungi or parasites.
  • They are the “search” battalion of the immune system՚s search-and-destroy system, tasked with finding an enemy and marking it for destruction.
  • When antibodies find their target, they bind to it, which then triggers a cascade of actions that vanquish the invader. Antibodies are part of the so-called “adaptive” immune system, the arm of the immune system that learns to recognize and eliminate specific pathogens
  • The two arms at the top of the antibody՚s Y shape bind to what՚s known as the antigen. The antigen can be a molecule, or a molecular fragment often some part of a virus or bacteria. (For instance, the new coronavirus SARS-CoV-2 has unique “spikes” on its outer coat, and some antibodies bind to and recognize these spike proteins.)
  • The bottom of the Y, or the stalk, binds to several other immune-system compounds that can help kill the antigen or mobilize the immune system in other ways.
Antibody or Immunoglobulins Antigen-Antibody Binding
  • An antibody is composed of two heavy chains (50 KD each) and two light chains (25 KD each) , which are joined by disulfide bonds to form a ‘Y’ shaped structure (150 KD) . Antibodies are further divided into two regions: a variable region and a constant region.
  • The variable region is responsible for the antigenic specificity of an antibody. This region includes a fragment antigen binding (Fab) portion that binds the antigen with high specificity. There are two Fab portions in each antibody, which can simultaneously bind two identical epitopes (a specific antibody-binding site of an antigen) of a particular antigen.
  • The constant region of an antibody includes a fragment crystallization (Fc) portion that binds cell surface receptors (Fc receptors) on circulating WBCs, macrophages, and natural killer cells. This binding is necessary to initiate an immune reaction. In addition, there are two hinge regions that join the Fab and Fc portions of an antibody.

Types of Antibodies

Types of Antibodies
  • Antibodies, which are also called immunoglobulins (Ig) , all have the same basic Y-shape, but there are five variations on this theme — called IgG, IgM, IgA, IgD and IgE
  • Immunoglobulin G, or IgG, is just one Y, whereas IgM looks a bit like the 10-armed Hindu goddess Durga, with five Ys stacked together, and each prong can bind one antigen.
  • IgG and IgM are the antibodies that circulate in the bloodstream and go into solid organs, Cyster said. IgA is “squirted out of the body,” in mucus or secretions, Cyster told Live Science. IgE is the antibody that typically triggers allergic responses, such as to pollen or peanuts. IgD has historically been enigmatic, but one of its roles is to help activate the cells that make antibodies.
  • IgM – 5 % in blood; 1st to be synthesized in response to antigen; large size
  • IgM typically exists as polymers of identical subunits, with a pentameric form as the prevalent one.
  • In its pentameric form, five basic antibody units are attached by disulfide bonds.
  • Due to its large size, IgM is mostly intravascular and has a lower affinity for antigens. However, since pentameric IgM has 10 antigen binding sites, it has higher avidity (overall binding strength) for antigens than IgG and acts as an excellent activator of the complement system and agglutination.
  • The first type of antibody to form after you are exposed to a virus is IgM, which emerges within 7 to 10 days after exposure. At about 10 to 14 days, the body begins making IgG, which is the immune system՚s “major workhorse.”
  • IgG – 70 – 75 % in blood; 4 types as IgG1, IgG2, IgG3, and IgG4
  • IgG is the main component of the humoral immune system (immune response initiated by macromolecules present in the extracellular fluid) because of its abundance. Proteins are responsible for triggering IgG1 and IgG3 production, whereas IgG2 and IgG4 typically respond to foreign polysaccharides.
  • Due to its small size (monomeric) and high diffusibility, IgG is the prevalent type in the extracellular fluid that binds Fc receptors on phagocytic or other lytic cells and initiates the antibody-dependent cell-mediated cytotoxicity (ADCC) response – a cell-mediated defense mechanism wherein effector cells (phagocytes) destroy the target cell
  • IgG triggers phagocytosis to initiate opsonization reaction – a process used to destroy foreign particles (e. g. , bacteria) through phagocytosis. Apart from these functions, IgG is the only antibody that can cross the placenta and provides passive immunity to the fetus and infants in the first few months of life.
  • IgA - 10 – 15 %- serum, nasal mucus, saliva, breast milk, and intestinal fluid
  • At mucosal surfaces, IgA provides the primary defense against inhaled and ingested pathogens.
  • It has two subtypes namely IgA1 and IgA2
  • IgE – Least Prevalent Common in Allergic Reactions
  • Concentration of IgE increases significantly in allergic conditions, such as bronchopulmonary aspergillosis, and parasitic diseases, such as schistosomiasis.
  • In response to pathogens, IgE binds to mast cells via specific receptors, followed by pathogen-mediated cross-linking of these receptors (degranulation) .
  • IgD – B cell antigen receptor
  • Participate in B cell maturation, maintenance, activation, and silencing. IgD may be involved in humoral immune responses by regulating B cell selection and homeostasis.
  • Camelid antibodies
  • Nanobodies with only heavy chains
  • Used in antibody-based therapies
  • Camelid (single-domain) antibodies, commonly known as nanobodies, are composed of only heavy chains (no light chains) . These antibodies were discovered in 1989 following the analysis of total and fractionated IgG molecules in the serum of a camel.
  • In Camelid antibodies, the antigen recognition site is composed of a single variable domain. These antibodies are potential candidates to be used in antibody-based therapies because of some special characteristics, such as smaller size, good solubility, high thermostability, high affinity and specificity, low immunogenicity, and higher tissue penetration rate.

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