The Immune System

A phagocyte is a type of white blood celll that carries out phagocytosis, which is the engulfment of pathogens. They're found in the blood and in the tissues and are the first cells to respond to a pathogen inside the body. This is how they work:

• A phogocyte recognises the antigens on the pathogen.
• The cytoplasm of the phagocyte moves around the pathogen , engulfing it.
• The pathogen is now contained in a phagocytic vacuole in the cytoplasm of the phagocyte.
• A lysosome (an organelle that contains lysosomal enzymes) fuses with the phagocytic vacuole. The lysosomal enzymes then break down the pathogen.
• The phagocyte then presents the pathogens antigens- it sticks the antigens on its surface to activate other immune system cells. 

T-Cells 

A T-cell is another type of white blood cell. It has proteins on its surface that bind antigens presented to it by phagocytes. This activates the T-cell. Some T-cells release substances to activate B-cells and some attach to antigens on a pathogen and kill the cell.

B-Cells

B-cells are also a type of white blood cell. They're covered with antibodies- proteins that bind antigens to form an antigen-antibody complex. Each B-cell has a different shaped antibody on its membrane, so different ones bind to different shaped antigens. When the antibody on the surface of a B-cell meets a complementary shaped antigen, it binds to it. This, together with the substances released from T-cells, activates the B-cells. The activated B-cell divides into plasma cells. 

Plasma cells make more antibodies to a specific antigen. 

Plasma cells are identical to the B-cell. They secrete loads of the antibody specific to the antigen. Antibody functions include:

• Coating the pathogen to make it easier for a phagocyte to engulf it. 
• Coating the pathogen to prevent it from entering host cells.
• Binding to it and neutralising toxins produced by the pathogen. 

Antibodies are proteins- they're made up of chains of amino acid monomers linked by peptide bonds. The specificity of an antibody depends on its variable regions. Each antibody has a different shaped variable region (due to different amino acid sequences) that's the complementary one specific antigen. The constant regions are the same in all antibodies. 

The immune system can be split into two- the cellular response and the humoral response. 

The Cellular Response

• The T-cells and other immune systems cells that they interact with, eg-phagocytes, form the cellular response.

The Humoral Response

• The B-cells and the production of antibodies form the humoral response.

When an antigen enters the body for the first time it activates the immune system. This is called the primary response. The primary response is slow because there aren't many B-cells that can make the antibody needed to bind to it.

Eventually the body will produce enough of the right antibody to overcome the infection. Meanwhile the infected person will show symptoms of he disease. After being exposed to an antigen, both T-cells and B-cells produce memory cells.

These memory cells remain in the body for a long time. Memory T-cells remember the specific antigen and will recognise it a second time around. Memory B-cells record the specific antibodies needed to bind to the antigen.

This person is now immune- their immune system has the ability to respond quickly to a second infection. 

If the same pathogen enters the body again, the immune system will produce a quicker, stronger immune response- this is the secondary response.

Memory B-cells divide into plasma cells that produce the right antibody to the antigen.

Memory T-cells divide into the correct type of T-cells to kill the cell carrying the antigen.

The secondary response often gets rid of the pathogen before you begin to show any symptoms.