The Immune System

Antigens - molecules (usually proteins) found on the surface of cells

Immune response:

1. Phagocytes engulf pathogens (phagocytosis)

- Phagocytes recognise antigens on the pathogen

- The cytoplasm of the phagocyte moves around the pathogen, engulfing it

- The pathogen is contained in a phagocytic vacuole

- A lysosome fuses with the phagocytic vacuole and lysosomal enzymes break down the pathogen 

- The phagocyte then presents the pathogen's antigens

2. Phagocytes activate T-cells

T-cell - type of white blood cell with proteins on the surface that bind to the antigens presented to it by phagocytes.

- This activates the T-cell

- Some T-cells release substances to activate B-cells

- Some T-cells attach to antigens on a pathogen and kill the cell

3. T-cells activate B-cells which divide into plasma cells

B-cells - type of white blood cell covered with antibodies (proteins that bind with 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 helps to activate the B-cell

- The activated B-cell divides into plasma cells

4. Plasma cells make more antibodies to a specific antigen

Plasma cells - identical clones of B-cells

-  They secrete loads of the antibody specific to the antigen. 

Antibody functions:

1. Coating the pathogen to make it easier for a phagocyte to engulf it

2. Coating the pathogen to make it harder to enter host cells

3. Binding to and neutralising toxins produced by the pathogen