The Immune System

Skin - provides a physical barrier to prevent entry of pathogens

Mucous Membranes/Goblet Cells - secrete mucus to trap pathogens

Ciliated Epithelium - waft and remove mucus containing parasites

Blood Clotting - prevents entry of pathogens to wounds

Lysozymes (not lysosomes) in tears, saliva and nasal secretions - kill bacteria

Stomach acid - kills pathogens

1. The phagocyte detects pathogens by the chemicals they give off.

2. The pathogens attach to the cell surface membranes of the phagocytes

3. The phagocyte engulfs the pathogen by an infolding of its membrane, forming a phagosome

4. The lysosomes inside the phagocyte fuse with the phagosome and release hydrolytic enzymes to break down the pathogen

5. Antigens from the pathogen may be displayed on the surface of the phagocyte (in the case of macrophages) to form antigen presenting cells. Harmless products of the digestion are absorbed by simple diffusion, and indigestible material is discharged from the phagocyte. 

There are two main components of the immune system: the humoral and the cell-mediated responses. They work both together and separately to protect the body from disease.

The humoral response is associated with the serum (the non-cellular part of the blood) and involves the action of antibodies secreted by B lymphocytes (plasma cells). Antibodies are found in extracellular fluids such as lymph, plasma and mucus. The humoral response protects against circulating viruses, and bacteria and their toxins. 

The cell-mediated response is associated with the production of T lymphocytes. It is most effective against bacteria and viruses located within host cells, as well as parasitic protozoa, fungi and worms. It is also an important defence against cancer, and is responsible for the rejection of implanted tissue. 

Both T and B lymphocytes develop from stem cells in the bone marrow of adults and the liver of foetuses. However T cells mature in the thymus, whereas B cells mature in the bone marrow. 

T Lymphocytes

1. Foreign antigens on APCs attach to complimentary-shaped receptors on the specific T cell. This is clonal selection.

2. The specific T cell undergoes clonal expansion, where it divides by mitosis. 

3. The daughter T cells defferentiate into T memory, T cytotoxic and T helper cells. 

4. Memory T cells remember the specific shape of the pathogen's antigens in case of a secondary infection. 

5. T cytotoxic (or T killer) cells release proteins called cytokines which attatch to the membranes of infected host cells, opening pores and allowing water to move into the infected cells by osmosis. This causes cells to burst (lysis) 

6. T helper cells attach to complimentary-shaped receptors on the specific B cell's membrane. This is clonal selection.

B Lymphocytes

1. After the T helper cells have activated the specific B cell during clonal selection, the B cell undergoes mitosis (clonal expansion).

2. The daughter B lymphocytes then differentiate into plasma cells and B memory cells

3. B memory cells remember the specific shape of the antibodies needed to combat the pathogen's antigens in case of a secondary infection. If a secondary infection occurs, memory B cells rapidly divide and become plasma cells.

4. The plasma cells secrete antibodies which have a shape complimentary to the shape of the specific antigens released by pathogens, and the shape of the antigens on the cell-surface membranes of pathogens. 

5. Antibodies combat pathogens by:

• binding to several pathogens and causing them to clump together. This is called agglutination. Phagocytes can then engulf and digest these clumps more quickly
• binding to toxins to neutralise them
• binding to the receptors of pathogens to stop them binding to, and infecting, body cells