immunology: the antibodies, atgventure

Immunology: the antibodies

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immunology: the antibodies, atgventureLet’s study antibodies, one of the most powerful weapons used by a special type of a body soldier called plasma cells. Antibodies, also called immunoglobulins (Ig) are Y-shaped Glycoproteins produced by a differentiated type of B-cells called plasma cell.

For better understanding, we advise that you read introduction to immunology; Immunology: first line of defense and immunology: second line of defense before continuing in this post.

How B-cells differentiate into plasma cells

B-cells are a special type of lymphocytes produced in the bone marrow together with the T-cells and other forms of white blood cells. Red blood cells are also produced in the bone marrow. B-cells mature in the bone marrow, unlike T-cells another type of lymphocytes that migrate to the thymus tissues for maturation.

B-cells, like the dendritic cells and macrophages, have an antigen-presenting ability. B-cells engulf antigens (bacteria) and present the antigens on their cells in form if MHC II (Major Histocompatibility Complex II) which is recognized by T-cells which does the final blow and lead to the elimination of antigen in the body. The binding of CD4+ T-cell to the B-cells leads to its activation and subsequent differentiation into memory B-cells and plasma cells

The memory B-cells store information about a particular antigen and cause elevated immune response upon subsequent encounter with such microorganism while the plasma cells secrete antibodies that fight microorganism.

Functions of antibodies

The major roles performed by antibodies in the body include:

  • Neutralization
  • Opsonisation

Antibodies neutralize the effectiveness of antigens (viruses and bacteria) by binding to their surfaces thus making them unable to attack blood cells.

Opsonization is also another function of antibodies. They bind to surfaces of antigens, mark and make it readily available for complement proteins and phagocytic cells which lyse and engulf (eat up) the bacteria antigen respectively.

Structure of Antibodies

immunology: the antibodies

The antibodies structure is referred to as dimers of dimers, containing two heavy chains and two light chains. The molecular weight of the heavy chains is 50000Da while the light chains are exactly half of the heavy chains. The molecular weight of light chains is 25,000Da

Antibodies are classified into five based on their heavy chains:

  1. IgG (Gamma heavy chains)
  2. IgM (Mu heavy chains)
  3. IgD (Delta heavy chains)
  4. IgE (Epsilon heavy chains)
  5. IgA (Alpha heavy chains)

While there are two types of light chains in the antibody:

  1. Kappa (κ) light Chains
  2. Lambda light (λ) chains

The carbohydrate residue of the antibodies gives antibodies its solubility. In the antibody is a region known as the hinge region that provide the antibody with some flexibility. It should, however, be noted that only three classes of antibodies possess hinge region: IgA, IgG and IgD while IgM and IgE possess no hinge region. Another important region is known as the fragment crystallizable (Fc) region (found at the tail end of the antibody structure, and help in interaction with phagocytic cells and complement proteins) as well as the Fab region (The region responsible for antigen-antibody binding).

The antibody structure is also made up of variable regions which are composed of amino acid. Antibody structure of the antibody is held together firmly by disulfide bridges

Classes of antibodies and their functions

  1. IgG possess Gamma (ᵞ) heavy chain. It is usually the second immune response to appear at a site of infection. It also helps to cause bacterial immobilizations and viral toxins neutralization. This is the only antibody that can cross the placenta and confer immunity to fetus.
  2. IgM possesses Mu (µ) heavy chain. It is the first antibody produced against antigens during infection
  3. IgD possesses Delta (ᵟ) heavy chain. It plays a major role in the maturation of B-cells.
  4. IgE is composed of Epsilon (ε) heavy chain (73,000 Da). Immunoglobulin E plays a vital role in hypersensitivity reactions. It is responsible for asthmatic reaction and anaphylactic shock. This antibody helps to protect the body against parasites.
  5. IgA possesses an Alpha (α) heavy chain. It is the Immunoglobulin predominantly found external secretions such as breast milk, saliva, tears and mucus of bronchial, genitourinary and digestive tracts. IgA can cross the epithelial layer and enter into body secretion and provides local immunity in GI tracts, respiratory tract, and genital tract. IgA also neutralizes viral toxins and inhibit their attachment on host surfaces.

Structurally, both IgG, IgD and IgE exist in monomeric form while Ig M exist in pentameric form and IgA in dimeric form.

Cite this article as: Fagbohun, S.O., "Immunology: the antibodies," in ATG Ventures, 23/02/2019, https://atgventure.com/immunology-the-antibodies/.

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