Question

How do antigens and antibodies differ?

Answer 1

They are both proteins ( or polysaccharides in some cases ) that you can find on the surface of a cell, but they play different roles in physiology.

Explanation:

Antigens are usually proteic or polysaccharidic molecules found on the surface of the cell which play an, let's say, " ID " role for the cell. There are two main types of antigens in the human body:

• MHC I
• MHC II
( Major Histocompatibility Complex )

MHC I antigens are found in every cell in the human body, in order to tell other cells that they aren't intruders.

MHC II antigens are found on the surface of the immune cells ( B and T lymphocites ) , so that other cells can identify them as a part of the immune system . Immune cells also have MHC I antigens, as they are cells of the human body.

Antibodies are proteic molecules, often represented in the form " Y " ( due to the arrangement of their peptidic chains ) which can play 2 major roles in the immune response :

(A) They can be used as receptors for pathogens in order to initiate the immune response. It happens like this:

1) A macrophage cell finds and eats a pathogen.
2) The macrophage cell exposes the pathogens antigenes on their MHC II molecules
3) A specialised lymphocite called " T helper cell " recognises the MHC II molecule and the foreign antigen( using the antibodies of the T helper cell), so if the cells talked it would go like this:

macrophage cell : Hey, i've found a pathogen ! Look at it ! (points at the MHC II with the foreign antigen)
T helper cell: ( identifying the cell as an immune cell due to the MHC II molecule and the foreign antigen) Oh, i see what you've got there! Let's initiate the immune response !

(B) They can be used to block/destroy the intruders.

After what happened at (A), the T helper cell starts to produce some specialised proteins called limphokines, which will play the role of the " postman ", lets say, in the immune response. Limphokines stimulate the B or T cells, depending on the nature of the intruder.

If the intruder is a virus, the B cells turn into "plasmocites" , meaning they start to produce antibodies ( ~2000/second ) that link to the foreign pathogens and destroy or inhibit their actions.