Distinguish between antigens and antibodies, active and passive immunity, and explain why the body doesn't develop full immunity to diseases like malaria.

a)

i) Distinguish between Antigen and antibody

• Antigen: A foreign substance, usually a protein or polysaccharide, that triggers an immune response in the body, leading to the production of antibodies.
• Antibody: A protein produced by plasma cells (a type of white blood cell) in response to an antigen. Antibodies specifically bind to antigens to neutralize them or mark them for destruction by other immune cells.

ii) Distinguish between Active immunity and passive immunity

• Active immunity: Immunity developed by an individual's own immune system after exposure to an antigen, either through natural infection or vaccination. It is long-lasting because memory cells are formed.
• Passive immunity: Immunity acquired by receiving antibodies from an external source, such as from a mother to her baby through breast milk or placenta, or via injection of antiserum. It provides immediate but short-term protection as no memory cells are produced.

b) The body does not develop full immunity against some diseases like malaria because the causative agents, such as the Plasmodium parasite, have complex life cycles and exhibit significant antigenic variation. This means the parasite can change its surface proteins, making it difficult for the immune system to recognize and mount a sustained, effective response against all forms of the pathogen.

c) Prevention is better than cure because it avoids the onset of disease, thereby preventing suffering, health complications, and potential long-term damage to the body. It is also often more cost-effective and requires fewer resources than treating an established illness, which can involve expensive medications, hospital stays, and prolonged recovery periods.

d) The mechanism of blood clotting involves several steps:

1. When a blood vessel is injured, platelets adhere to the damaged site and release chemicals that attract more platelets, forming a temporary platelet plug.
2. Damaged tissue cells and activated platelets release clotting factors.
3. These clotting factors initiate a cascade of reactions that ultimately convert inactive prothrombin into active thrombin.
4. Thrombin then acts as an enzyme to convert soluble plasma protein fibrinogen into insoluble fibrin threads.
5. Fibrin threads form a mesh-like network that traps red blood cells and more platelets, forming a stable blood clot that seals the wound and stops bleeding.