The previous article was about the different types of immune cells. This article is about a special molecule in immunity known as immunoglobulin.

There might be a question that what is so special about this immunoglobulin. There is a reason for this. These molecules play an important and inevitable role in the phagocytosis of pathogens. To understand this, it is essential to know about immunoglobulins.

The immunoglobulin is a gamma globulin, a specialized group of proteins (glycoprotein) produced in response to pathogens. It is produced by the plasma cells (a globulin protein present in the plasma). These constitute 25-30% of the blood proteins.

There are two important terms that are more commonly known by the most, they are the antigen and the antibody. The antigen is the molecule present on the surface of the pathogen and can stimulate an immune response. There is a small part of the antigen called the epitope which interacts with the antibody.  The epitope is known as the antigen determinant site. An antigen can have unlimited epitopes.

On the contrary, the antibody is the molecule produced in response to the antigen in order to kick it away. The part of the antibody which interacts with the antigen is called a paratope. An antibody must have at least 2 paratopes. These antibodies belong to the immunoglobulins. All antibodies are immunoglobulins but not immunoglobulins are antibodies. To understand how the antibody helps in immunity, it is essential to understand the structure of an antibody/immunoglobulin. The image below shows the general structure of an immunoglobulin:-

There are two chains in an immunoglobulin namely the light chain and heavy chain. The light chain has 212 amino acids (the building block of protein) and the heavy chain has 450 amino acids. Each chain has two types namely the constant and variable. These regions are based on the amino acid sequences. Half of the light chain (1 out of 2) is constant and the rest is variable. A quarter of the heavy chain (1 out of 4) is variable and the rest is constant. These are linked by two types of sulfide bonds namely the intra (H-H AND L-L) and inter (H-L). These molecules contain carbohydrates (CHO) hence these are called as glycoproteins.

The tip of the variable regions of the heavy and light chain is hypervariable in nature and these constitute the antigen-binding site (Fab). These are hyper-variable because they have to produce amino acid sequences complementary to that of the antigen so that they can interact together. The other site is called a crystallizable region (Fc).

Having known all this, now it will be convenient to explain the process by which the antibody plays in the prevention of infections.

There are millions of substances that pass through the blood every day. So there must be a criterion/substance to identify them whether they are pathogenic. This is where antigen comes to play. These antigens present on the surface of the pathogens alert the immune system which then identifies this as a pathogen. So in response to the antigen, a suitable antibody is secreted and deployed to the target site. On reaching the antigen, the Fab region binds with the antigen.

The ultimate aim of the immune system is to abolish the pathogen and one way is by phagocytosing them. This is done by the macrophages. But it is essential for them to identify the substance before engulfing it. This is where the antibody comes to play. The Fc region of the antibody combines with the receptor of the macrophage. This facilitates the process of phagocytosis.

Hence the antibody acts like a bridge between the source (antigen) and the destination (macrophage) aiding in phagocytosis. This is essential because in most of the cases the macrophages, it is difficult to identify the non-self-objects and this is where antibody helps.

In the case of the new pathogen, the antigen is new, and therefore their might not be a suitable antibody. In that case, the macrophage cannot phagocytocise the pathogen and it reigns in the body causing infection and disease.

The next article is about the types of immunoglobulins.