AS Biology OCR - The Immune System

What Antigens Do ?

When a pathogen (like a bacterium) invades the body, the antigens on its cell surface are identified as foreign, which activates cells in the immune system.

Antibodies Help to Clear an Infection - Agglutinating Pathogens

- Each antibody has two binding sites, so an antibody can bind to two pathogens at the same time, the pathogens become clumped together. - Phagocytes then bind to the antibodies and phagocytose a lot of pathogens all at once. - Antibodies that behave in this way are known as agglutinins.

Antibodies Help to Clear an Infection - Neutralising Toxins

- Like antigens, toxins have different shapes. - Antibodies called anti toxins can bind to the toxins produced by pathogens. - This prevents the toxins from affecting human cells, so the toxins are neutralised. - The toxin antibody complexes are also phagocytosed.

Immune Response

- The immune response involves specific and non-specific stages. - The non-specific response happens in the same way for all microorganisms, whatever foreign antigens they have. - The specific response is antigen-specific it is aimed at specific pathogens. It involves white blood cells called T and B lymphocytes.

Structure of Antibodies

- The variable regions of the antibody form the antigen biding sites. The shape of the variable region is complementary to a particular antigen. The variable regions differ between antibodies. - The hinge region allows flexibility when the antibody binds to the antigen. - The constant regions allow binding to receptors on immune system cells. The constant region is the same in all antibodies. - Disulfide bridges hold the polypeptide chains of protein together.

Antibodies Help to Clear an Infection - Preventing the Pathogen Binding to Human Cells

- When antibodies bind to the antigens on pathogens, they may block the cell surface receptors that the pathogens need to bind to the host cells. - This means the pathogen can't attach to or infect the host cells.

Four Main Stages in the Immune Response - Phagocytes Activate T Lymphocytes

1) A T lymphocyte is another type of white blood cell. 2) Their surface is covered with receptors. 3) The receptors bind to antigens presented by APCs. 4) Each T lymphocyte has a different receptor on its surface. 5) When the receptor on the surface of a T lymphocyte meets a complementary antigen, it binds to it, so each T lymphocyte will bind to a different antigen. 6) This activates the T lymphocyte, this process is called clonal selection. 7) The T lymphocyte then undergoes clonal expansion, it divides to produce clones, which the differentiate into different types of T lymphocytes that carry out different functions. 8) T lymphocytes can become T helper cells, T killer cells, T regulatory cells and memory cells.

Four Main Stages in the Immune Response - Phagocytes Engulf Pathogens

1) A phagocyte recognises the antigens on a pathogen. 2) The cytoplasm of the phagocyte moves round the pathogen, engulfing it. This may be easier by the presence of opsonins. 3) The pathogen is now contained in a phagosome in the cytoplasm of the phagocyte. 4) A lysosome fuses with the phagosome. The enzymes break down the pathogen. 5) The phagocyte then presents the pathogen's antigens. It sticks the antigens on its surface to activate other immune system cells. When a phagocyte does this it is acting as an antigen presenting cell (APC).

Four Main Stages in the Immune Response - T lymphocytes Activate B lymphocytes, Which Divide Into Plasma Cells

1) B lymphocytes are another type of white blood cell. 2) They're covered with proteins called antibodies. 3) Antibodies bind to antigens to form an antigen antibody complex. 4) Each B lymphocyte has a different shaped antibody on its surface. 5) When the antibody on the surface of a B lymphocyte meets a complementary shaped antigen, it binds to it, so each B lymphocyte will bind to a different antigen. 6) This, together with substances released from T helper cells, activates the B lymphocyte. This process is another example of colonial selection. 7) The activated B lymphocyte divides, by mitosis, into plasma cells and memory cells. This is another example of colonial expansion.

Cell Signalling

1) Cell signalling is basically how cells communicate. 2) A cell may release (or present) a substance that binds to the receptors on another cell, this causes a response of some kind in the other cell. 3) Cell signalling is really important in the immune response because it helps to activate all the different types of white blood cells that are needed.

Four Main Stages in the Immune Response - Plasma Cells Make More Antibodies to a Specific Antigen

1) Plasma cells are clones of the B lymphocyte (they're identical) 2) They secrete loads of the antibody, specific to the antigen, into the blood. 3) These antibodies will bind to the antigens on the surface of the pathogen to form lots of antigen antibody complexes.

Phagocyte

A phagocyte is a type of white blood cell that carries out phagocytosis (engulfment of pathogens). They're found in the blood and in tissues and carry out a non-specific immune response.

Antigens

Antigens are molecules (usually proteins or polysaccharides) found on the surface of cells.

Neutrophils

Neutrophils are a type of phagocyte. They're the first white blood cells to respond to a pathogen inside the body. Neutrophils move towards a wound in response to signals from cytokines. The cytokines are released by cells at the site of a wound.

Opsonisns

Opsonins are molecules in the blood that attach to foreign antigens to aid phagocytosis.

Primary Response

Pathogen - Enters for 1st Time Speed of Response - Slow Cells Activated - B and T lymphocytes Symptoms - Yes

Secondary Response

Pathogen - Enters for 2nd Time Speed of Response - Fast Cells Activated - Memory Cells Symptoms - No

Cell Signalling - Example

T helper cells release interleukins that bind to receptors on B lymphocytes. This activates the B lymphocytes, the T helper cells are signalling to the B lymphocytes that there's a pathogen in the body.

T Killer Cells

These attach to antigens on a pathogen and kill the cell

T Helper Cells

These release substances to activate B lymphocytes

T Regulatory Cells

These suppress the immune response from other white blood cells. This helps to stop immune system cells from mistakenly attacking the host's body cells.