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How does the immune system protect the body from infectious germs?

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Immune stimulation by activated helper T cells — immune system Stimulation of immune response by activated helper T cells. Activated by complex interaction with molecules on the surface of a macrophage or some other antigen-presenting cell, a helper T cell proliferates into two general subtypes, T_H1 and T_H2. These in turn stimulate the complex pathways of the cell-mediated immune response and the humoral immune response, respectively.

The immune system protects the body from infectious germs through a coordinated, two-tiered defense strategy: innate immunity and acquired immunity.1

Innate Immunity

This is the body's first line of defense, present from birth. It includes physical barriers like the skin and chemical barriers such as enzymes in saliva. If germs breach these barriers, specialized white blood cells called phagocytes (such as neutrophils and macrophages) engulf and destroy invaders, while natural killer cells target infected cells.123

Acquired Immunity

When innate defenses are insufficient, the body mounts a targeted response using lymphocytes:
* B cells: Upon recognizing a specific antigen, B cells mature into plasma cells that secrete antibodies. These proteins bind to germs, neutralizing them or marking them for destruction by other immune components.14
* T cells: Cytotoxic T cells destroy infected or foreign cells, while helper T cells coordinate the overall response.13

Crucially, some lymphocytes become memory cells, allowing the immune system to recognize and eliminate the same pathogen much faster upon future exposure.15