Innate Immunity: The First Line of Defense
Innate immunity is your body's non-specific defense system. It is "innate" because you are born with it, and "non-specific" because it attacks all foreign invaders in a similar manner.
Physical and Chemical Barriers
The first goal of innate immunity is to prevent entry.
- Physical Barriers: The epidermis provides a keratinized, multi-layered physical shield. Cilia in the respiratory tract act as microscopic hairlike projections to sweep substances away, while mucus traps foreign particles.
- Chemical Barriers: Lysozyme (found in tears and saliva) breaks down bacterial cell walls. Sebum and other skin secretions create an acidic environment that inhibits microbial growth. Additionally, healthy microbiota compete with pathogens for resources.
The Inflammatory Response and Phagocytosis
If a pathogen breaches the barriers, the body triggers inflammation—a non-specific response to tissue damage.
- Chemical Signaling: Damaged cells or mast cells release histamine, which causes vasodilation and increased capillary permeability. Prostaglandins are also released at sites of infection to promote inflammation.
- Phagocytosis: Specialized cells called phagocytes (including macrophages, neutrophils, and dendritic cells) rush to the site to ingest and digest pathogens.
- Complement System: This group of proteins enhances the response by inducing cytolysis (bursting the cell) or opsonization (coating the pathogen to make it easier for phagocytes to "grab").
Fever and Cytokines
When the infection is systemic, the body may induce a fever. This is triggered by pyrogens:
- Endogenous pyrogens: Proteins like interleukin-1 produced by your own immune cells.
- Exogenous pyrogens: External substances like lipopolysaccharides from Gram-negative bacteria.
Exam Tip: Remember the difference between an antipyretic and an anti-inflammatory. Acetaminophen is an antipyretic (fever reducer) and analgesic (pain reliever), but it is not an anti-inflammatory. Drugs like NSAIDs (aspirin, ibuprofen) are anti-inflammatory because they inhibit prostaglandins.
Adaptive Immunity: The Specialized Strike Force
When innate immunity is insufficient, the adaptive immune system takes over. This system is highly specific and creates "memory" so that future encounters with the same pathogen are handled more efficiently.
Immune Cells and Maturation
Adaptive immunity relies on two main types of lymphocytes:
- B Cells: These cells mature in the bone marrow. When activated by an antigen, they differentiate into plasma cells, which are the "factories" that secrete antibodies.
- T Cells: These cells mature in the thymus.
Antigen Presentation and T Cell Activation
For T cells to work, they must be "shown" the enemy. This is done via Antigen-Presenting Cells (APCs), such as macrophages.
- Helper T Cells (CD4+): These are the "generals" of the immune system. They use CD4 receptors to recognize antigens presented on MHC-II molecules by APCs. Once activated, they secrete interleukin-2 (IL-2), a cytokine that stimulates the rapid proliferation of both B and T cells.
- Cytotoxic T Cells (CD8+): These are the "assassins." They use CD8 receptors to recognize antigens presented on MHC-I molecules (which are found on almost all infected body cells). They destroy the target cell by releasing perforin (to create holes) and granzymes (to trigger apoptosis/cell death).
Key Concept: Immunological Tolerance
To prevent the body from attacking itself, lymphocytes must exhibit self-recognition—the ability to recognize your own MHC proteins. This ensures immunological tolerance, meaning lymphocytes lack reactivity to your own proteins.
Immunoglobulins (Antibodies)
Antibodies are proteins secreted by plasma cells to neutralize or mark pathogens for destruction. There are five main classes:
| Class | Primary Function & Location |
|---|
| IgG | Most abundant; protects against bacteria/viruses; crosses the placenta to protect newborns |
| IgA | Found in secretions (saliva, tears, mucus, breast milk) |
| IgM | The first antibody secreted during an initial exposure to an antigen |
| IgD | Found primarily on the surfaces of B cells as antigen receptors |
| IgE | Involved in allergic reactions and defense against parasitic worms |
Types of Immunity
Immunity can be categorized based on how you acquired it and whether your body produced the response itself.
1. Active vs. Passive
- Active Immunity: Your body produces its own antibodies and memory cells after exposure to an antigen. This provides long-lasting protection.
- Passive Immunity: You receive "ready-made" antibodies from another source. This provides immediate but temporary protection because no memory cells are created.
2. Natural vs. Artificial
- Naturally Acquired Active: Getting sick (e.g., catching a cold) and your body building immunity.
- Naturally Acquired Passive: Antibodies passed from mother to baby via the placenta (IgG) or breast milk (IgA).
- Artificially Acquired Active: Receiving a vaccination, which introduces antigens to stimulate an immune response without causing disease.
- Artificially Acquired Passive: Receiving an injection of prepared immunoglobulins (antibodies).
