11.1 The Immune System

📖 Main Content

🔬 Section 1: Phagocytes — The Body's First Defenders

What are phagocytes?

When harmful microorganisms (called pathogens — things like bacteria and viruses that cause disease) enter your body, your immune system fights back. The first type of cell to respond is called a phagocyte.

• Phagocytes are a type of white blood cell — special blood cells whose job is to defend your body.
• They are produced continuously in the bone marrow (the soft tissue inside your bones).
• They are part of the non-specific immune response, meaning they attack any pathogen they detect — they do not target one specific type.
• Their main job is to destroy pathogens and remove dead or damaged cells.
• The process by which phagocytes engulf and destroy pathogens is called phagocytosis (pronounced "fag-oh-sy-TOH-sis"). Think of it like a cell "eating" a pathogen.

There are two main types of phagocyte, and you need to know both:

1. Neutrophils
2. Macrophages

Type 1: Neutrophils

Neutrophils are the most common type of phagocyte. Here is how they work, step by step:

Step 1 — Attraction (Chemotaxis)

• When pathogens invade your body, they release certain chemicals. Your body's own damaged cells also release chemicals (such as histamine — a warning signal released by injured cells).
• These chemicals act like a trail of breadcrumbs that attract neutrophils toward the site of infection.
• Moving toward a chemical signal like this is called chemotaxis (pronounced "kee-mo-TAX-sis").

Step 2 — Recognition and Attachment

• Once a neutrophil reaches the pathogen, receptor proteins on its surface attach to antigens on the surface of the pathogen. (Antigens are explained fully in Section 2.)
• Think of this like a lock and key — the receptor fits the antigen.

Step 3 — Endocytosis (Engulfing the pathogen)

• The cell surface membrane of the neutrophil folds outward and wraps around the pathogen, completely surrounding it.
• The pathogen becomes trapped inside a bubble-like structure inside the neutrophil called a phagocytic vacuole (also called a phagosome). Think of it as the pathogen being swallowed inside a bag.
• This process of engulfing something by folding the membrane around it is called endocytosis.

Step 4 — Fusion with a Lysosome

• Inside the neutrophil, there are small organelles called lysosomes (pronounced "LY-so-soams"). These are like tiny bags of powerful digestive enzymes — chemicals that can break down and destroy biological material.
• The lysosome fuses (joins) with the phagocytic vacuole. The combined structure is called a phagolysosome.

Step 5 — Killing and Digestion

• The digestive enzymes (for example, proteases — enzymes that break down proteins) are released into the phagolysosome and destroy the pathogen completely.

Step 6 — Neutrophil death

• After destroying the pathogen, the neutrophil itself dies.
• Pus (the yellowish fluid you sometimes see in an infected wound) is made up of large numbers of dead neutrophils.