Structure of Antiboody
Many pathogens multiply in the extracellular spaces of the organisms’ bodies. These extracellular spaces are protected by B lymphocytes which are responsible for humoral immunity.
B lymphocytes originate from bone marrow and they also complete their maturation in bone marrow. After maturation, these cells are released into the blood and keep circulating in the blood, lymph and secondary lymphoid tissues. When these B cells recognize specific antigens, they get activated. This recognition occurs via specific receptors present on B cells which are known as B cell receptors or BCR. Once activated, these B cells proliferate and differentiate into effector B cells and memory B cells.
Effector B cells are called plasma cells and they secret antibodies. They produce and secret antibodies specific to antibodies recognized by the B cells.
What are the Antibodies?
Antibodies are the glycoproteins produced and secreted by plasma cells. Antibodies are also known as immunoglobulins. This is because they belong to a group of glycoproteins called globulins. The term immune reflects that these glycoproteins play a major role in immunity.
The basic structure of antibodies and B cell receptors is the same so B cell receptors are also known as immunoglobulins. The difference is that the B cell receptor is a membrane-bound immunoglobulin whereas antibodies are secreted immunoglobulins.
The Basic Structure of Antibody
The basic structure of an antibody resembles an alphabetical letter “Y”. Each antibody consists of four polypeptide chains, two identical heavy chains also known as H chains and two identical light chains known as L chains. The terms heavy and light refer to the molecular weights of the chains. Since these are polypeptides, the N terminal of polypeptide chains is present at the tip end while the C terminal is present at the base of polypeptide chains.
Light Chains of Antibody
Each light chain has less molecular weight of about 25 Kilo Daltons and has 220 amino acids. Each light chain is connected to a heavy chain via a disulfide bond. Two types of light chains named as the kappa light chain and the lambda light chain are present in humans. Both have the same structure and functions but these are encoded on different chromosomes. The Kappa chain is encoded on chromosome 2 whereas the lambda chain is encoded on chromosome 22.
Heavy Chains of Antibody
Heavy chains have high molecular weights of up to 50 to 70 Kilo Daltons and have 440amino amino acids in each heavy chain. Five types of heavy chains are found in humans which are named gamma, alpha, mu, epsilon and delta. These all are encoded on chromosome 14.
The heavy chains are connected by two disulfide bonds in the mid-region. Besides these disulfide bonds, several non-covalent bonds are also present which keep the heavy chains together.
Regions of the Antibody
Hinge Region
The mid-region of heavy chains has considerable flexibility. This region is known as the hinge region. It makes the antibody able to adjust to different spatial arrangements of antigens. In other words, the hinge region makes the rotation and bending of an antibody molecule.
Fc Region
The bottom region is called the Fc region. This is named so because the C in Fc stands for crystallizable. Fc region is constant means it does not change its shape. Due to this region, the antibody is involved in fixing the complement cascade. This region can also bind with macrophages and mast cells. Furthermore, it has the function of controlling isotype.
Fab Region
The top region is known as the Fab region. Since it contains an antigen-binding site, it is named the Fab region. Fab region is the variable region of antibody means its genetic composition can change. The Fab region is also responsible for the idiotype of antibody which is the ability of an antibody to bind with various antigens.
There are two antigen-binding sites in each antibody molecule which are formed by the first 110 amino acids of the N terminal regions of both light and heavy chains. The variability of these amino acid sequences in these regions is localized within certain regions. Since these regions consist of variable amino acids, they are called hypervariable regions or complementarity-determining regions or CDRs.
Functions of Antibodies
Opsonization
Opsonization is the process applied for the enhancement of phagocytosis. Antibodies usually float in the blood plasma. When an antigen enters the body, the antibodies bind to the antigen. After interacting with the antigen, the antibodies increase the rate of phagocytosis.
Phagocytic cells have receptors for the Fc regions of antibodies. Through these receptors, the antibodies bind to the phagocytic cells. After forming the antigen-antibody complex, the antigen-bound antibodies now bind to the phagocytes which destroy the pathogens. In this way, antibodies kill any pathogen entered into the body. This function is performed by the heavy chain constant regions of antibodies.
Neutralization
Neutralization is the binding of antibodies to the antigens to neutralize pathogens such as bacteria, viruses and toxic substances.
Antibodies bind to the antigens present in pathogens and form an antigen-antibody complex. This complex makes the antigens inactive or neutralized so that the antigen cannot bind to the receptors present in the host cells and will not cause any harm to the host cells. Ig A antibodies are mostly involved in the neutralization process.
Activation of the Complement System
Antibodies such as the subclasses Ig M and Ig G can activate the complement system when they recognize any pathogen in the body. The activation of the complement system results in the degradation of pathogens. The complement proteins make pores in the cells of pathogens which are then filled with the host body fluids and burst.
Antibody-Dependent Cell-Mediated Cytotoxicity
In the case of virus-infected cells and tumor cells, the infection is eliminated by natural killer cells or NK cells, but these NK cells cannot recognize the pathogens directly. So first, the antibody binds to the receptors of the targeted cells, and then it binds to the NK cells. The NK cells have receptors for the Fc portion of the antibody. Due to the binding of NK cells to the antibody-bound targeted cells, the NK cells release their granules, ultimately leading to apoptosis or killing of targeted cells.
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