The Role of Blood in Defense

Blood is vital in the immune system as it contains various components that work to protect the body. This note focuses on Specific Objective 4.5 of Section B of the CSEC Biology Syllabus, which asks us to describe the role of blood in defending the body against disease, with emphasis on the clotting mechanism, the role of phagocytes and natural immunity.

The Clotting Mechanism

One such way the blood protects the body is through clotting, which prevents the loss of blood and entry of pathogens that would otherwise occur when wounds are formed.

Terms to know:

Platelets- These are tiny fragments of cells in the blood that activate when a blood vessel is damaged.

Clotting factors- These are various proteins in the blood plasma that initiate a series of reactions that lead to the formation of a clot.

Steps in clotting once a wound is formed:

Step 1: Platelets stick to the site of injury and form a temporary plug.

Step 2: Platelets release chemicals that activate prothrombin (an inactive protein) into thrombin (an active enzyme).

Step 3: Thrombin then converts fibrinogen (a soluble protein) into fibrin (insoluble threads).

Step 4: Fibrin forms a mesh that traps blood cells, forming a clot which eventually hardens into a scab.

The Role of Phagocytes in Defense

Phagocytes are a type of white blood cell involved in the destruction of pathogens.

Steps in Phagocytes destroying pathogens:

1. Detection: Phagocytes identify foreign pathogens such as bacteria and viruses through chemical signals.
2. Engulfment: They surround and engulf the pathogen in a process called phagocytosis.
3. Digestion: The pathogen is enclosed in a vesicle and is subsequently destroyed by digestive enzymes.

Natural Immunity

Natural immunity can be defined as the body’s inherent ability to fight off pathogens without any prior exposure to them.

Components of natural immunity include:

• Skin and mucous membranes which are the first line of defense that prevents entry of pathogens.
• White blood cells which include phagocytes and lymphocytes.
• Antimicrobial substances such as chemicals in blood such as complement proteins that destroy pathogens.

Role of Lymphocytes in Natural Immunity:

Lymphocytes are a type of white blood cells which bind to antigens (foreign proteins) on the surface of pathogens.

There are two types: B-cells which produce antibodies to neutralize pathogens or mark them for destruction, and T-cells, which actually destroy the pathogens.