When barriers are breached, your immune system sends in the cavalry — leukocytes (white blood cells). These are your body's soldiers, and each type has a specialized role.

Granulocytes — The Rapid Response Team

Named for the visible granules in their cytoplasm, these cells are ready for immediate action.

Neutrophils (50-70% of leukocytes) — The most abundant white blood cell and first responders to infection. They're phagocytes (cell eaters) that engulf and destroy bacteria. They're short-lived (hours to days) and die in large numbers at infection sites — pus is largely dead neutrophils!

Eosinophils (1-4%) — Specialists in fighting parasites (worms) and involved in allergic reactions. Their granules contain enzymes that attack parasitic worms too large for phagocytosis.

Basophils (0.5-1%) — The least common white blood cell. They release histamine (causes blood vessel dilation) and heparin (prevents clotting). Important in allergic responses.

Agranulocytes — The Specialists

These cells lack visible granules and have more specialized roles.

Monocytes (2-8%) — The largest blood cells. When they leave the bloodstream and enter tissues, they transform into macrophages — "big eaters" that phagocytose pathogens and debris. They also present antigens to T cells, bridging innate and adaptive immunity.

Lymphocytes (20-40%) — The cells of adaptive immunity. B cells produce antibodies; T cells (helper and cytotoxic) coordinate immune responses and kill infected cells. Some become memory cells that remember past infections.

Natural Killer (NK) cells — These lymphocytes are part of innate immunity. They patrol for cells that look "wrong" — virus-infected cells or cancer cells — and induce them to self-destruct (apoptosis).

Phagocytosis is the process by which cells engulf and digest foreign particles, pathogens, or cellular debris. It's like cellular Pac-Man — the cell surrounds the target, swallows it, and digests it.

The Main Phagocytes

Neutrophils: The most abundant phagocyte, first to arrive at infection sites. They're like the SWAT team — fast, aggressive, but short-lived. They die after engulfing a few bacteria.

Macrophages: "Big eaters" derived from monocytes. They're larger, live longer, and can engulf many pathogens. They also clean up dead cells and debris. Tissue-specific names: Kupffer cells (liver), microglia (brain), alveolar macrophages (lungs).

Dendritic cells: Found in tissues that contact the external environment (skin, respiratory tract). They phagocytose pathogens and then travel to lymph nodes to show T cells what they found — they're the "scouts" that activate adaptive immunity.

The Process of Phagocytosis

1. Chemotaxis: Phagocyte follows chemical trail to infection site (like a bloodhound tracking a scent)

2. Adherence: Phagocyte attaches to the pathogen's surface

3. Ingestion: Phagocyte extends membrane around pathogen, engulfing it and forming a phagosome (a bubble containing the pathogen)

4. Digestion: Lysosomes fuse with phagosome, releasing digestive enzymes and reactive oxygen species

5. Killing: Pathogen is destroyed

6. Exocytosis: Undigested material is expelled from the cell

Opsonization — Making pathogens "tastier"

Antibodies and complement proteins can coat pathogens, marking them for phagocytosis. This is like putting a "eat me" sign on bacteria — phagocytes recognize the coating and engulf the pathogen more readily.

Inflammation is your body's response to tissue injury or infection. It's not a disease — it's a protective mechanism that isolates the problem and recruits help.

The Cardinal Signs of Inflammation

Doctors since ancient Rome have recognized four (now five) signs:

1. Rubor (Redness) — Caused by increased blood flow to the area

2. Calor (Heat) — Warm blood flowing to the area raises local temperature

3. Tumor (Swelling) — Fluid leaking from blood vessels into tissues (edema)

4. Dolor (Pain) — Chemicals released by immune cells stimulate nerve endings

5. Functio laesa (Loss of function) — A protective mechanism; you rest the injured part

What Causes These Signs?

When tissue is damaged, resident macrophages release inflammatory mediators:

Histamine — Released from mast cells and basophils. Causes vasodilation (widening of blood vessels) and increased vascular permeability (leaky vessels).

Prostaglandins — Cause pain and fever. Aspirin and ibuprofen work by blocking prostaglandin production.

Cytokines — Signaling molecules that recruit more immune cells to the area.

The Process

1. Tissue injury occurs

2. Damaged cells and resident macrophages release inflammatory mediators

3. Blood vessels dilate (redness, heat) and become leaky (swelling)

4. Neutrophils and monocytes exit blood vessels and migrate to injury site

5. Phagocytes clean up pathogens and debris

6. Tissue repair begins

Why Inflammation Can Be Harmful

While acute inflammation is protective, chronic inflammation can damage tissues. Diseases like rheumatoid arthritis, atherosclerosis, and inflammatory bowel disease involve chronic inflammation where the immune system attacks the body's own tissues.

Different immune cells work together like musicians in an orchestra, each playing their part at the right time.

The Innate Response (First Responders)

Neutrophils — Arrive within minutes to hours. They're the infantry, engaging the enemy immediately but dying in the process. Pus at an infection site is mostly dead neutrophils.

Macrophages — Arrive slightly later and stay longer. They continue phagocytosis, clean up dead neutrophils and debris, and release cytokines that coordinate the response.

NK cells — Patrol for cells displaying abnormal markers (virus-infected or cancerous) and induce them to undergo apoptosis (programmed cell death).

The Adaptive Response (Specialists)

Dendritic cells — Bridge innate and adaptive immunity. They collect samples from infection sites and travel to lymph nodes to activate T cells.

T helper cells — The conductors of the immune orchestra. They release cytokines that activate B cells, T cytotoxic cells, and macrophages. Without T helpers, the adaptive response collapses (as in HIV/AIDS).

T cytotoxic cells — The assassins. They find cells infected with viruses or intracellular bacteria and kill them by inducing apoptosis.

B cells — Transform into plasma cells that produce antibodies specific to the pathogen. Antibodies mark pathogens for destruction.

Memory cells — B and T cells that remember the pathogen, enabling faster future responses. This is the basis of vaccines.