An immune response occurs when the body recognizes and defends against harmful substances or organisms (antigens).
Here's a breakdown of the process:
1. Recognition of the Antigen
- Antigens are substances, usually proteins or polysaccharides, on the surface of cells, viruses, fungi, bacteria, and some nonliving substances such as toxins, chemicals, and foreign particles.
- Innate Immunity: The body's first line of defense, involving physical barriers (skin, mucous membranes), chemical barriers (stomach acid, enzymes), and internal defenses (inflammation, fever, natural killer cells, phagocytes). These defenses are non-specific and respond to a wide range of threats.
- Adaptive Immunity: This is a specific response that develops over time. The body "learns" to recognize specific antigens and mounts a targeted attack. Key players are lymphocytes: B cells and T cells.
2. Activation of Immune Cells
- B Cells: When a B cell encounters an antigen that matches its specific antibody, it's activated. Activated B cells differentiate into:
- Plasma cells: These cells produce large amounts of antibodies specific to the antigen.
- Memory B cells: These provide long-term immunity by "remembering" the antigen for future encounters.
- T Cells: There are several types of T cells:
- Helper T Cells (Th): These cells help activate B cells and cytotoxic T cells. They also release cytokines, which signal other immune cells and coordinate the immune response.
- Cytotoxic T Cells (Tc or Killer T cells): These cells directly attack and destroy infected or cancerous cells displaying the antigen.
- Regulatory T Cells (Treg): These cells help to suppress the immune response after the threat has been neutralized, preventing autoimmunity.
3. Antibody Production
- Antibodies (immunoglobulins) are Y-shaped proteins produced by plasma cells. They circulate in the blood and lymph and bind specifically to the antigen that triggered their production.
- Mechanism of Action: Antibodies neutralize antigens through various mechanisms:
- Neutralization: Blocking the antigen's ability to infect or damage cells.
- Opsonization: Coating the antigen to make it easier for phagocytes (e.g., macrophages, neutrophils) to engulf and destroy it.
- Complement Activation: Triggering the complement system, a cascade of proteins that can directly kill pathogens, enhance inflammation, and promote opsonization.
4. Cell-Mediated Immunity
- Cytotoxic T cells directly kill infected or cancerous cells that display the antigen on their surface.
- Mechanism of Action: The cytotoxic T cell binds to the infected cell and releases substances that trigger apoptosis (programmed cell death) in the target cell.
5. Regulation and Resolution
- The immune response is tightly regulated to prevent excessive inflammation and damage to healthy tissues.
- Regulatory T cells help to suppress the immune response once the antigen has been cleared.
- After the infection is cleared, most of the activated immune cells die off, leaving behind memory cells that provide long-term immunity.
Summary
In essence, the immune response is a complex series of events that involves recognizing foreign invaders (antigens), activating specialized immune cells (B and T lymphocytes), producing antibodies, directly attacking infected cells, and regulating the response to prevent harm to the body itself. This coordinated response protects the body from a wide range of threats and provides long-lasting immunity against previously encountered pathogens.
