The immune system

Antigens are molecules, usually proteins, that generate an immune response. They are found on the surface of cell surface membranes of prokaryotes and eukaryotes, and on the protein coat of viruses.

Phagocytes are a type of white blood cell that destroys organisms with foreign antigens via phagocytosis.

The steps to phagocytosis are:

1) The phagocyte recognises the foreign antigens on a pathogen.

2) The phagocytes engulf it, containing it in a phagocytic vacuole (phagosome) in its cytoplasm.

3) Lysosomes fuse with the phagocytic vacuole, releasing lysozymes which break down the pathogen.

4) The phagocyte presents the pathogen’s antigens on its surface to activate other immune system cells, acting as an antigen presenting cell.

There are two other types of white blood cells that need to be studied. These are T-cells and B-cells. 

After a phagocyte presents a pathogen’s antigens on its surface, it finds a T-cell with complementary receptor proteins on its surface and binds to it. This sensitises the T-cell allowing them to help fight against other pathogens with antigens complementary to the receptor proteins on the T-cell’s surface. There are two types of T-cells with different functions. These are:

1) Cytotoxic T-cells. These cells kill abnormal and foreign cells.

2) Helper T-cells. These cells release chemicals which activate and stimulate more phagocytes, as well as B-cells.

B-cells are covered in proteins called antigen-binding sites, which can bind to antigens to form an antigen-antibody complex. Each B-cell has different shaped antigen-binding sites. After a B-cell binds to a complementary antigen and has been activated by helper T-cells, they undergo a process called clonal selection which produces many plasma cells.

Plasma cells are clones of the original B-cell and they produce many monoclonal antibodies. Antibodies bind to the surface of pathogens, forming many antigen-antibody complexes. Antibodies have two binding sites, and thus can bind two pathogens together. As a result, the pathogens clump together (agglutination), which makes it easier for phagocytes to engulf and destroy them.  

The structure of an antibody is shown below:

antibody structure, beginnerbiology

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