INNATE IMMUNE SYSTEM (First Line of Defense)

These cells respond immediately when bacteria enter the body.

1) Neutrophils (Most Important Bacterial Killers)

These are the first responders and the most abundant white blood cells.

How they kill bacteria:

• Phagocytosis (engulf and digest bacteria)

• Release reactive oxygen species (ROS) like hydrogen peroxide

• Release enzymes (lysozyme, proteases)

• Form NETs (Neutrophil Extracellular Traps) — sticky DNA webs that trap bacteria

Where they act:

• Blood

• Sites of infection

• Pus is mostly dead neutrophils

They’re short-lived but extremely aggressive.

2) Macrophages

Large “big eater” cells.

Functions:

• Phagocytosis

• Destroy bacteria inside lysosomes

• Release inflammatory cytokines (IL-1, TNF-α)

• Present antigens to T cells (bridge to adaptive immunity)

Types:

• Tissue macrophages (Kupffer cells in liver, microglia in brain, etc.)

They’re slower than neutrophils but longer-lived.

3) Monocytes

• Circulate in blood

• Become macrophages when entering tissues

Think of them as macrophage precursors.

4) Dendritic Cells

Not mainly killers — but VERY important.

What they do:

• Capture bacteria

• Process bacterial antigens

• Activate T cells in lymph nodes

They start the adaptive immune response.

5) Natural Killer (NK) Cells

Mostly kill virus-infected cells, but can help in bacterial infections too.

Mechanism:

• Release perforin (creates holes)

• Release granzymes (trigger apoptosis)

They don’t directly eat bacteria, but kill infected host cells.

ADAPTIVE IMMUNE SYSTEM (Specific & Powerful)

This system develops memory.

6) B Cells

They don’t directly kill bacteria — but produce antibodies.

Antibodies help by:

• Opsonization (mark bacteria for phagocytosis)

• Activating complement system

• Neutralizing toxins

• Agglutination (clumping bacteria)

Important antibody types against bacteria:

• IgG

• IgM

• IgA (mucosal surfaces)

7) Plasma Cells

Activated B cells that produce massive amounts of antibodies.

8) Helper T Cells (CD4⁺)

Do not directly kill bacteria.

But they:

• Activate macrophages

• Help B cells make antibodies

• Release cytokines (IFN-γ boosts macrophage killing)

Very important for intracellular bacteria.

9) Cytotoxic T Cells (CD8⁺)

Kill cells infected with intracellular bacteria (like tuberculosis).