Structure and function of individual parts of immunoglobulins. Classes of immunoglobulins, properties and functions. Monoclonal antibodies - preparation, use.

Immunoglobulins are class of proteins produced by B cells and plasma cells. Another term used to refer to Immunoglobulins are antibodies. They serve as line of defense by binding to antigens that correspond to the pathogens that invade our body.

Structure and function of Immunoglobulins : Basic structure

Antibodies are Y shaped glycoproteins consisting of two small polypeptide chains and two large polypeptide chains. In each respective pair, the polypeptide chains are identical. There are five major classes of immunoglobulins, of which the most abundant in human sera are gamma globulins.

Each short arms of the Y shaped glycoprotein are composed of a pair of chains; light chain and heavy chain held together by disulfide bonds. The two arms are held together by one or more disulfide bonds by the hinge. The chains in immunoglobulins have similar tertiary structure with certain features, most notably "Immunoglobulin folds". Immunoglobulin folds are characterized "beta barrel" antiparallel beta pleated sheets connected by loops of variable and which are held together disulfide bond/s.

The Antibodies can be divided into two fragments;

• Fc: a homogenous portion which can be crystallized
• Antigen binding region: Fab2: this region of the antibody binds to antigens
  • This region consist of heavy and one of two types of light chains.

Basic structure of antibody

Specific features of antibodies

• Hinge regions: this regions provides the characteristic of flexibility that further enables the antibody to bind to the antigen
• In Fab, each heavy chain binds one of two different kinds of light chains; kappa and lambda
• There are five types of heavy chains recognized in humans and each of them are assigned a greek letter
  • Alpha Ά
  • Mu μ
  • Epsilon ε
  • Delta δ
  • Gamma γ

• Variable domain : which consists of heterodimers of light chain and heavy chain and the function of this region is antigen binding
  • Two identical variable regions are present in Fab region thus allowing a complete Ig to bind atleast 2 two identical antigens.
  • Antibodies binds to antigens via non covalent bonds such as hydrogen bonds, Van der Waals forces, hydrophobic bonding and electrostatic interactions.
    

    Highlights the variable regions on Antibodies
• Hyper variable regions : Constituents of these region are 100-110 amino acids.
  • Complementary determine region : present on each heavy and light chain, each chain contains three hyper variable regions, they are called that because their structure is complementary to target antigen
  • This feature allows Ig to recognize a wide range of antigens

• Constant domains : the function they serve to provide structural stability
  • In light chains, one constant can be seen.
  • However heavy chains are seen to have 3 - 4.
  • Fc regions are a result of the association between these domains which can elicit different downstream reactions depending on isotype.

Classes of immunoglobulins

• There are five isotopes of antibodies, they differ from each based on the type of heavy chain present in Fab.
  • Immunoglobulin G : Gamma γ
  • Immunoglobulin A : Alpha Ά
    • they are well known for their role in mucosal immune response
    • their dimeric form is found in bodily fluids like saliva, tears and sweat
    • Secretory component : the attached portion after cleavage of polymeric Ig receptor from dimer IgA
    • Known to works against bacteria particularly against Gram negative and agglutinate viruses
    • Passive immunity for neonates and infants as they are found in breastmilk
    • There are two forms of IgA ->
      • IgA1 and IgA2 can differentiated from each other when compared by the number of disulfide bonds and length of hinge reaction
      • In sera, there are significantly higher levels IgA1 as opposed to mucosal secretion which contain equimolar levels of IgA1 and IgA2
  • Immunoglobulin M : Mu μ
    • On mature or naive B cells, they are expressed as membrane bound
    • They are produced and secreted in plasma cells (activated form of B lymphocytes)
    • Play an important role in primary immune response against bacteria
    • During a inflammatory reaction, IgM is the first antibody isotope secreted
    • They typically exist as a pentameric form as they bind each to each other via the interaction between J chain
    • Each pentameric complex can bind up to 10 antigens
    • These antibodies are released before affinity maturity is developed
    • Lower affinity for antigens than IgG and IgA.
  • Immunoglobulin E : Epsilon ε
    • physiological scenarios, the amount present in sera would at be trace levels.
    • They are predominantly in monomeric form
    • They play an important role in the activation of mast cells
    • The threshold amount of IgE required to trigger an inflammatory response.
    • They play a role in defense against parasites particularly helminths
    • They play an important role in deterring pathogens that enter via epithelial barriers
    • A disadvantage of IgE is that it can be triggered by substances that are abundantly present in our environment that are not necessarily harmful like pollen
  • Immunoglobulin D : Delta δ
    • they are typically found bound to membrane of B cells and expressed in monomeric form.
    • In human sera, it it typical found low levels

Monoclonal antibodies

Monoclonal antibodies are clonal copies of antibody that binds to one specific antigen. They are known for their participation in diagnose and treatment of certain diseases.

Classification of Antibodies

Classification depends on the way these antibodies are produced. They can be produced in four different ways

• When monoclonal antibodies are produced within mice they are called Murine.
  • example : Rituximab can be used in treat in certain cancer that originate from the blood and other autoimmune disease
• When the monoclonal antibodies are not solely murine and contain more human components, they are called Chimeric.
  • example : to treat inflammatory diseases like RA (rheumatoid arthritis) such as Infliximab
• When a majority of the proteins used for monoclonal antibodies are of human origin but there's a small fraction obtained from mice, they would be referred to Humanised.
  • Trastuzumab is a type mAB that used in treatment of certain type of breast cancer (HER2 positive)
• When monoclonal antibodies are of complete human origin, these are called Human.
  • similar to infliximab, Adalimumab have same spectrum of action particularly auto immune disease like RA and psoriasis .

Applications of Monoclonal Antibodies (mAB)

• Diagnostic Testing : for certain toxins, drugs or hormones
• Pregnancy Testing : Monoclonal antibodies have been used detect HCG (human chorionic gonadotropin) are now present in pregnancy test kits
• Oncology
  • Radio immunodetection of cancer : a diagnostic method used to detect the presence of cancerouscells, the mAB are usually radio-labelled antibodies
  • Radioimmunotherapy of cancer : a therapeutic method that involves the use of mAB to treat various kinds of cancer.
    • An example : Ritoxin in 1997 was approved by the FDA for commercial used
• Treatment of viral disease : theres research surrounding the use of mAB in the therapy related to viral disease that lead to complications like AIDS
• Identifying Pathogens : There's an increase use of mAB used to identify a particular strain of bacteria
• Identification of particular cells and their functions : mAB can be used to track certain cells or molecules and used to determine its function
• Predict organ rejection : OKT3 is an example mAB against the antigen T3 that facilitates in alleviation of the effect and determine likelihood of organ rejection during organ transplant.
• Immunization against Rhesus disease : Anti-RH D immunoglobulins that has been used prevent hemolytic disease of the fetus and newborn (HDFN) caused by Rh incompatibility. However mAB against RHD is considered to be a safer as opposed to polyclonal serum of human origin.