Specific Immune Response, A2 Biology, Edexcel

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• Antigen (any molecule the body recognises as not being of its own self) enters the body. Most antigens, such as the ones found on the surface of bacteria or viruses, are protein 

• The antigens' large size and characteristic molecular shape allow the lymphocytes to identify which ones are foreign (non-self)

• B and T cells respond to foreign antigens

• Foreign body engulfed by macrophage (phagocytosis)

• Complementary receptors on the surface of B cells bind to non-self antigens and become antigen-presenting cells (same way as macrophages do)

• Macrophage becomes an antigen-presenting cell (APC). Proteins produced by cells are continually added to and removed from the fluid mosaic cell surface membrane. When a piece of biological material is engulfed by a macrophage, protein fragments (peptides) from the material become attached to proteins in the cell. These are added to the macrophage's cell surface membrane, where they're displayed as 'non-self' antigens. They act as a signal to alert the immune system to the presence of foreign antigens in the body.

• A T helper cell with complementary-shaped receptors, called CD4 receptors, on its surface binds to the antigen on the surface of the antigen-presenting cells

• Once activated by this binding,  each T helper cell divides to produce a clone of active T helper cells and a clone of T memory cells

• The T memory cells remain in the body for months or years. If an individual is exposed to the same antigen in the future their immune system can respond more quickly.

• Activated T helper cells produce cytokines (chemical signals) which stimulate B cells to divide. They also enhance the activity of phagocytes and stimulate division of T killer cells.

• Activated T helper cell with complementary receptor binds to APC and produces cytokines (proteins) that stimulate the B cell.

• Under the influence of cytokines, the B cell divides/differentiates to produce 2 clones of cells. (clonal selection)

• B effector cells - these differentiate to produce plasma cells

• B memory cells - like T memory cells, these cells are longer-lived. They remain for months or years in the body, enabling an individual to respond more quickly to the same antigen in the future

• The first time a B cell comes across a foreign antigen that is complementary to its cell surface receptors, the production of sufficient antibody producing cells by clonal selection takes about 10-17 days. 

• During this time, the person will suffer symptoms of the infection

• Plasma cells secrete antibodies into the blood and lymph. Plasma cells are relatively short-lived, lasting only a few days

• Antibodies are special protein molecules of a class known as immunoglobulins. They consist of 4 polypeptide chains, bonded by disulphide bridges,  creating a Y shape. The amino acid sequence in the lower part of the Y shape varies little between different types of antibody. This is known as the constant region. The amino acid sequence and shape of the binding site is different in each type of antibody. The shape of the binding site is complementary to the shape of the specific antigen. There are many different types of binding sites to fit many different shapes of antigens.

• Antibodies bind to the antigens on the microbe cell surface membrane. They act as labels.

• The antibody then binds to the antibody receptor on the surface of the macrophage

• The macrophage engulfs the antibodies and the foreign body

• Lysosomes fuse with the vacuole, releasing digestive enzymes that destroy the foreign body

• Lymphocytes are white blood cells that circulate in the blood and lymph, and gather in large numbers at the site of any infection

• Reserve supplies of lymphocytes are held in strategically positioned lymphoid tissue

• Body cells become infected by foreign bodies

• If a bacterium or virus infects a body cell, a fragment of antigen is presented on the cell surface membrane in the same way as occurs with macrophages. Cell becomes an APC

• T killer cells with complementary receptors bind to the antigen presented on the cells.

• The T killer cells divide to form an active clone; this division is stimulated by the cytokines from T helper cells. Without cytokines, there would not be enough T killer cells to fight a viral infection.

• The T killer cells release enzymes that create pores in the membrane of infected cells. This enables ions and water to flow into the infected cell, which swells and bursts (undergoes lysis). The pathogens within the cell are released.

• Once out of the cell they can be labelled by antibodies from B cells as targets for destruction by macrophages

• Clones of T killer memory cells are also made through division

• T lymphocytes are produced in the bone marrow and mature in the thymus gland

• T cells each have one specific type of antigen receptor on their surface. This only binds to an antigen with the complementary shape.

• 1. Immature T cells are produced by division of stem cells in the bone marrow

• 2. Immature T cells move to the thymus via the blood

• 3. T cells mature in the thymus

• 4. Mature T cells leave the thymus in the blood and move to the lymph nodes and the spleen.

• As lymph fluid passes through a lymph node, T cells are activated by any pathogens present.

• As blood passes through the spleen, T cells are activated by any pathogens present

• B cells are produced in the bone marrow

• Each B cell has one specific type of antigen receptor on its surface. The B cells is activated when its receptor binds to an antigen with the complementary shape.

• Each B cell binds to only one specific antigen. A microbe usually has several different types of antigen on its surface. Each different antigen will bind and activate different B cells.

• During early human embryo development, 100 million different B cells are produced in the bone marrow.

• Each of these divides rapidly to produce clones of these cells, providing the baby with an immune system that can respond to a tremendous variety of antigens that might invade its body after birth.

• If infected by the same bacterium or virus again, the immune system responds much faster

• The secondary immune response involves memory cells and only takes about 2-7 days

• The B memory cells produced in the primary response can differentiate immediately to produce plasma cells and release antibodies

• The invading virus or bacteria are often destroyed so rapidly that the person is unaware of any symptoms

• The person is said to be immune