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Antigen vs Pathogen – What is the Difference?

Antigen vs Pathogen – What is the Difference?

  • Pathogen –

We all might have experienced falling sick such as having fever or infection. In very simple term if I put it, the organism which has the ability to make us sick is called pathogen. An organism that can cause disease is called pathogen. Pathogens can be bacteria, viruses, fungi and even protozoa. Pathogens are foreign organisms which mean they are different than bacteria found in our body or on our skin (normal flora). Normal flora is friendly bacteria but pathogens are not, pathogens invade our body and cause disease.

When pathogen enters our body we would expect our immune system to fight against it. But do you know that our immune system or immune cells cannot recognize pathogen as whole. What they can recognize is a specific part (molecule) of a pathogen present on its surface. These specific molecules present on surface of pathogen and recognized by immune system is called antigen. 

  • Antigen – Antibody generator

Antigen means Antibody generator. It is a molecule located on the cell wall of bacteria or on coat of virus or on any other organism. Antigens are generally proteins but they can also be polysaccharides, nucleic acid or lipids.

These antigens present on pathogens are very specific for that particular pathogen. Such as an Ag present on Mycobacterium tuberculosis is unique to it and is different than an Ag present on Salmonella.

One more important thing to know about Ag is that the term Ag is not restricted to these surface molecules only. An Ag can be any foreign molecule which triggers immune response in our body. Such as MHC molecule from a different individual in organ transplant, some protein in food, component of serum etc.

So the major difference over here that we need to understand is Pathogen is a whole organism which causes the disease but Ag is part of this organism which is actually recognized by our immune system. 

Figure – Difference between Antigen and Pathogen

I hope this post helps 🙂

To understand more details, watch a video on this topic here.

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Antibody – Structure Overview

Antibody – Structure Overview

In this post we will discuss the structure of antibody. We will try and understand all the terms related to antibody structure and why are they named so. Let’s begin 🙂

Antibody = is a globular protein which is synthesized by plasma B-cells in respond to foreign antigen.

Because antibody is globular protein with immune function, it is also known as Immunoglobulin.

So when we talk about the structure of antibody, this Y looking figure (figure a) comes in our mind. This figure (a) is the typical structure of an antibody. Now we will see each of the labels one by one and see why are they named so.

Figure a – Structure of Antibody

  1. Heavy chain and Light chain – antibody is made up of two heavy chains and two light chains.Why are they named so – because of their molecular weight. That means heavy chain is more in molecular weight than light chains. Each heavy chain weighs 50 kD (kilodalton) whereas each light chain weighs 25 kD (kilodalton).Therefore, molecular weight of one single molecule of antibody = 150 kD (50 kD + 50 kD + 25 kD + 25 kD, because as mentioned before each antibody is made up of two heavy chains and two light chains). This is for one single molecule of antibody which means if you talk about IgA or IgM, molecular weight will be more because IgA is a dimer and IgM is pentamer.Types of heavy chains are µ, γ, α, δ and ε and light chains are κ and λ. Any of these heavy and light chains will be present in different types of antibody. The class of antibody is determined based on type of heavy chain present in it.That means heavy chain µ gives IgM, heavy chain γ gives IgG, α gives IgA, δ gives IgD and ε gives IgE. Let me tell you guys how to easily remember which heavy chain gives what antibody. Just look at the first letter of each heavy chain and there is the answer.Therefore, µ – Mu –IgM,γ – Gamma – IgG,α – Alpha – IgA,δ – Delta – IgD,ε – Epsilon – IgE
  2. Variable and Constant region –
    As you can see in figure (a) the top most subunits in heavy as well light chains are labeled as VH and VL respectively.
    Why are they named so – because of the high variability of amino acid sequence present in these regions. Therefore, VH is variable heavy chain and VL is variable light chain. Similarly, other subunits are labeled as CH and CL meaning constant heavy chain and constant light chain because there is no much change in the amino acid sequence.
  3. Antigen Binding site –
    It is present in the variable region. It is the specific amino acid sequence present in the variable region which can recognize the antigen and are called Complementarity Determining Region (CDR). CDR is also called Paratope. You must have read in immunology books that paratope of antibody binds to an epitope of antigen (epitope is part of antigen which can be recognized by paratope).
  4. Glycosylation site –
    There is no clear reason known for sites of glycosylation in antibody but one of the possible reasons says that as we know antibody is protein compound and it might contain hydrophobic amino acids which decreases its solubility in blood plasma. And we know that glycosylation increases the protein solubility so antibody solubility increases in blood plasma.
  5. Fab and Fc region
    Why are they named so – when the antibody structure was studied, it was digested by different enzymes which digested the antibody near hinge region giving two subunits. Upper subunit was named as Fab – Fragment antigen binding because this was the portion of antibody which binds to an antigen. Lower subunit was named as Fc – Fragment crystalizable and it is named so because when this fragment was stored it started crystalizing.
  6. Hinge region
    It gives the flexibility to antibody so that it can bind with an antigen in the best way possible.

So, these are the points we should know when we talk about the structure of antibody.

I hope this blog was helpful 🙂

 
Watch a video on this topic here.