Blood Cells and Immunity: Structure, Function, and Defense Mechanisms

Blood Cells and Their Functions

Red Blood Cells (Erythrocytes)

Red blood cells (RBCs) are specialized cells responsible for the transport of oxygen throughout the body. Their unique structure and composition enable efficient oxygen delivery to tissues and removal of carbon dioxide.

• Haemoglobin: A red, iron-containing protein found in RBCs. It binds with oxygen to form oxyhaemoglobin.

• Oxygen Transport: As RBCs pass through the lungs, they load (bind) oxygen. When they reach active tissues, they unload (release) oxygen for cellular respiration.

• Key Reaction:

$\text{haemoglobin} + \text{oxygen} \rightarrow \text{oxyhaemoglobin}$

• Example: In the lungs, high oxygen concentration favors the formation of oxyhaemoglobin; in tissues, low oxygen concentration causes its dissociation, releasing oxygen.

White Blood Cells (Leukocytes)

White blood cells are a diverse group of cells that play a crucial role in defending the body against infections and foreign invaders. There are several types, each with specialized functions.

• Main Role: Protection against pathogens (disease-causing microorganisms such as bacteria and viruses).

• Two Main Defense Mechanisms:

  • Phagocytosis (by phagocytes)

  • Production of antibodies (by lymphocytes)

Phagocytes

• Definition: White blood cells that ingest and destroy microorganisms by a process called phagocytosis.

• Mechanism: Phagocytes change shape and extend their cytoplasm to form pseudopodia, which surround and engulf pathogens.

• Proportion: About 70% of white blood cells are phagocytes.

• Example: Neutrophils are a common type of phagocyte that rapidly respond to infections.

Lymphocytes

• Definition: White blood cells responsible for the production of antibodies.

• Proportion: Approximately 25% of white blood cells are lymphocytes.

• Function: Produce antibodies, which are soluble proteins that circulate in the plasma and target specific pathogens.

• Antigens: Chemical markers on the surface of pathogens recognized by antibodies.

• Mechanism: Antibodies bind to antigens, marking pathogens for destruction.

• Example: B lymphocytes produce antibodies against viruses such as influenza.

Immunity

Primary and Secondary Immune Responses

The immune system can "remember" previous encounters with pathogens, leading to a faster and more effective response upon subsequent exposures.

• Memory Cells: Some lymphocytes become memory cells, remaining in the blood for years or even a lifetime.

• Secondary Immune Response: Upon re-exposure to the same antigen, memory cells trigger a rapid and robust production of antibodies, often preventing disease development.

• Comparison: The secondary response is faster and produces more antibodies than the primary response.

• Example: After recovering from chickenpox, memory cells provide long-term immunity against future infections.

Vaccination

Vaccination is a method of inducing artificial immunity by exposing the immune system to antigens without causing disease. This prepares the body to respond quickly to future infections by the actual pathogen.

• Mechanism: Injection of an agent containing antigens stimulates lymphocytes to produce antibodies and memory cells.

• Types of Vaccines:

  • Weakened (attenuated) microorganisms: e.g., vaccines for polio, tuberculosis (TB), and measles.

  • Dead microorganisms: e.g., vaccines for typhoid and whooping cough.

  • Modified bacterial toxins (toxoids): e.g., tetanus and diphtheria vaccines.

  • Purified antigens: Only the antigens are used, not the whole organism.

  • Genetically engineered vaccines: Bacteria engineered to carry antigens from other pathogens.

• Example: The measles vaccine contains a weakened form of the measles virus, prompting the immune system to develop immunity without causing illness.

Summary Table: Types of White Blood Cells and Their Functions

Type of White Blood Cell	Main Function	Proportion (%)	Mechanism
Phagocytes	Ingest and destroy pathogens	~70%	Phagocytosis using pseudopodia
Lymphocytes	Produce antibodies and memory cells	~25%	Antibody production; immune memory
Other (e.g., basophils, eosinophils)	Various roles in immune response	~5%	Release of chemicals, allergic responses

Additional info: The table includes a row for other white blood cell types (basophils, eosinophils) for completeness, though not explicitly mentioned in the original notes.