Which of the following helps protect the human body from infectious diseases and foreign invaders?

The human body constantly faces threats from bacteria, viruses, fungi, and other harmful microorganisms. To protect itself, the body has a defense system known as the immune system, and one of its most important components is the leukocytes, commonly called white blood cells (WBCs).

Leukocytes circulate in the bloodstream and body tissues to detect and destroy foreign invaders that can cause infections or diseases. They are produced in the bone marrow and are present in different forms, each with a specific protective role. The main types include neutrophils, lymphocytes, monocytes, eosinophils, and basophils.

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  Neutrophils are the first responders that attack bacteria and fungi.

  
  


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  Lymphocytes include B cells that produce antibodies and T cells that destroy infected cells.

  
  


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  Monocytes turn into macrophages and digest harmful particles.

  
  


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  Eosinophils fight parasites and control allergic reactions.

  
  


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  Basophils release chemicals like histamine that trigger inflammation to alert other immune cells.

  
  

In contrast, red blood cells (RBCs) carry oxygen throughout the body but do not play a role in fighting infections. Endoplasmic reticulum (ER) and Golgi apparatus are cell organelles that help in protein synthesis and transport, not in immune defense.

Leukocytes continuously monitor the bloodstream and tissues for signs of infection. When they detect an invader, they work together to destroy it and remember it for future protection. This ability to recognize and respond quickly to pathogens keeps the human body safe from many diseases.