Biology - AQA - Unit 2 - C2. The Immune Response

1. Phagocytosis
   A phagocyte (e.g. a macrophage) is a type of white blood cell that carries out phagocytosis (engulfment of pathogens). They’re found in the blood and in tissues and are the first cells to respond to an immune system trigger inside the body

• A phagocyte recognises the foreign antigens on a pathogen.

• The cytoplasm of the phagocyte moves round the pathogen, engulfing it.

• The pathogen is now contained in a phagocytic vacuole in the cytoplasm of the phagocyte.

• A lysosome fuses with the phagocytic vacuole. The lysozymes break down the pathogen.

• The phagocyte then presents the pathogen’s antigens - it sticks the antigens on its surface to activate other immune system cells. The phagocyte is acting as an antigen-presenting cell.

2. T-Cells
   A T-cell (also called a T-lymphocyte) is another type of white blood cell.

• It has receptor proteins on its surface that bind to complementary antigens presented to it by phagocytes. This activates the T-cell.

Different types of T-cells respond in different ways. For example:

• Helper T-cells (TH cells) release chemical signals that activate and stimulate phagocytes and cytotoxic T-cells (T cells), which kill abnormal and foreign cells.

• T cells also activate B-cells, which secrete antibodies

B-cells (also called B-lymphocytes) are also a type of white blood cell. They’re covered with antibodies-proteins that bind to 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 substances released from helper T-cells, activates the B-cell. This process is called clonal selection. The activated B-cell divides into plasma cells.

Tip: The B-cell divides by mitosis, so that all the cells produced are genetically identical. This means that they all produce identical (monoclonal) antibodies specific to the pathogen.

4. Antibody production
   Plasma cells are identical to the B-cell (they’re clones). They secrete loads of antibodies specific to the antigen. These are called monoclonal antibodies. They bind to the antigens on the surface of the pathogen to form lots of antigen-antibody complexes .

An antibody has two binding sites, so can bind to two pathogens at the same time. This means that pathogens become clumped together - this is called agglutination. Phagocytes then bind to the antibodies and phagocytose many pathogens at once. This process leads to the destruction of pathogens carrying this antigen in the body.

Antibodies are proteins—they’re made up of chains of amino acids. The specificity of an antibody depends on its variable regions, which form the antigen binding sites. Each antibody has a variable region with a unique tertiary structure (due to different amino acid sequences) that’s complementary to one specific antigen. All antibodies have the same constant regions.

• Cellular - The T-cells and other immune system cells that they interact with, e.g. phagocytes, form the cellular response.

• Humoral - B-cells, clonal selection and the production of monoclonal
  antibodies form the humoral response.

Both types of response are needed to remove a pathogen from the body and the responses interact with each other, e.g. T-cells help to activate B-cells, and antibodies coat pathogens making it easier for phagocytes to engulf them.

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 the disease.

After being exposed to an antigen:

• both T- 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 round.

• Memory B-cells record the specific antibodies needed to bind the antigen. The 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.

• Clonal selection happens faster.

• Memory B-cells are activated and divide into plasma cells that produce the right antibody to the antigen.

• Memory T-cells are activated and 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