The immune system is our body’s natural defense network. It keeps us safe from harmful bacteria, viruses, and other pathogens. To do this effectively, it uses a division of labor, where different cells and organs have specialized roles. Let’s learn about this immune system overview in four key areas: organs, cell types, lineages, and the CD labeling system.
Organs: Primary vs Secondary
The immune system relies on specialized organs where immune cells develop and mature & function.
- Primary Organs:
These act as the factory and training centers.- Bone marrow → the birthplace of most immune cells. It produces B cells, which later make antibodies, and also myeloid cells such as macrophages and neutrophils.
- Thymus → the training school for T cells. Immature T cells migrate here and learn to distinguish between “self” and “non-self” before they can function properly.
- Secondary Organs:
These are the battlefields where immune responses take place.- Lymph nodes trap antigens and provide a meeting ground where immune cells can interact with pathogens and each other.
- The spleen filters the blood, removing old red blood cells and detecting blood-borne pathogens.
- Gut-associated lymphoid tissue (GALT) provides immunity in the digestive system, which is constantly exposed to microbes.
Immune system overview—Immune Cells: Innate vs Adaptive
The immune system has two major branches that complement each other.
- Innate Immune Cells:
These are the first responders. They act quickly and non-specifically against invaders. They don’t need prior exposure to pathogens.- Examples: macrophages (engulf and destroy microbes), neutrophils (fast-acting infection fighters), and dendritic cells (sentinels that alert adaptive cells).
- Adaptive Immune Cells:
These are the specialized fighters. They undergo genetic rearrangements to create unique receptors that recognize specific antigens. Importantly, they also form memory, so the body can respond faster during future infections.- B cells produce antibodies to neutralize pathogens.
- T cells come in different forms: Helper T cells (coordinate the immune response) and Cytotoxic T cells (kill infected cells).
🧬 Lineage: Myeloid vs Lymphoid
All immune cells start from stem cells in the bone marrow but then split into two major family lines:
- Myeloid Lineage:
- Mostly innate cells.
- Includes macrophages, neutrophils, eosinophils, basophils, and dendritic cells.
- These cells provide broad, fast defense and are found throughout the body.
- Lymphoid Lineage:
- Mostly adaptive cells.
- Includes B cells and T cells, which undergo training in primary organs and activation in secondary organs.
- Interestingly, not all lymphoid cells are adaptive. Natural Killer (NK) cells belong to the lymphoid family but act as innate killers, destroying infected or cancerous cells without prior exposure.
🏷️ The “CD” Label
Scientists needed a way to classify immune cells based on surface molecules, so they created the Cluster of Differentiation (CD) system.
- What it means: CD numbers identify molecules on the surface of immune cells.
- How it works: The numbers simply show the order of discovery, not size, importance, or function.
- Example:
- CD4 → Found on T helper cells, acts as a co-receptor, and is a key target for HIV.
- CD28 → Discovered later, plays a role in activating T cells.
- Example:
- Why it can be confusing:
Some molecules have both a CD name and a common name. For example, CD25 is also known as the IL-2 receptor alpha chain.
✅ Key Takeaways: Immune system overview
- Primary organs = development and training of immune cells.
- Secondary organs = activation and fighting.
- Innate cells act immediately, adaptive cells are specialized and have memory.
- Myeloid lineage produces innate cells; lymphoid lineage produces adaptive cells (plus NK cells).
- CD labels classify immune cells but only indicate discovery order, not function