Pathogens have proteins on their surface that our immune system has learned to recognise as foreign. These proteins are called antigens. T cells, B cells & Macrophages all have the ability to recognise an antigen and once this has happened, they will trigger an immune response.
In addition to this, macrophages have the ability to present foreign antigens to T and B cells. Once a pathogen has been engulfed and destroyed MHC proteins inside the Macrophage stick to the pathogenic antigen. They are then incorporated into the cell membrane of the Macrophage, so it can present the foreign antigen and activate the T and B cells responses.
Antibodies (also called Immunoglobulins) are proteins produced by B cellls. They are found in blood plasma, lymph, tissue fluid, tears, mucus and milk.
Figure 6.35 – A simplified diagram of an antibody. The antigen with a complementary shape can bind to the antibody’s antigen binding site.
Each B cell produces a different immunoglobulin molecule which recognises and binds to a specific antigen. There are over a million different B cells in your body, therefore you have the ability to recognise and react to a million different antigens.
The variable region of the immunoglobulin protein is what recognises & binds to the antigen. Each variable region is different, hence the name.
There are 5 different families of immunoglobulin molecule in the human body (G, M, A, D &
- IgG – also known as g-globulin). The families can be distinguished from each other by slight differences in the constant region of the protein
Each antobody molecule contains two pairs of proteins;
- Two heavy chains
- Two light chains
Each pair of chains is held together by disulphide bridges (hydrogen bonds would be too weak).
Each immunoglobulin molecule has 2 antigen binding sites and can, therefore, bind 2 antigens at one time. This means that a single antibody molecule can bind to 2 pathogens at the same time, which causes pathogens to clump together and form the Antibody-Antigen Complex.
The formation of the Antibody-Antigen Complex is important because it;
- Isolates pathogens so they cannot infect other host cells
- Makes it easier for macrophages to engulf & destroy the
- Stops the pathogen from entering a host cell
- Makes it easier for T cell activation as more antigens are presented in one area