Immunology Chapter: Innate vs Adaptive Immunity

Learning Objectives

• Compare and contrast innate and adaptive immunity
• Describe the role of T cells and B cells in immunity
• Describe local inflammation

Innate Immunity (Nonspecific)

• Local response
• Protects against a wide variety of pathogens and toxins
• Present from birth
• Doesn't differentiate between types of pathogens (e.g., bacteria, viruses, dust, bugs)
• Includes external and internal defenses
• Epithelial barriers
• High acidity of secretions
• Phagocytosis
• Inflammation, fever

Nonspecific Phagocytosis

• Neutrophils: First to arrive at an infection site
• Macrophages: Organ-specific phagocytes in liver, spleen, lymph nodes, lungs, and brain.
  • Line sinusoids of liver, spleen, and lymph nodes to remove pathogens.
  • Chemokines released from tissue attract phagocytes through a process called chemotaxis
  • Phagocytes move out of capillaries through fenestrations via diapedesis

Interferons

• Polypeptides produced by cells infected with a virus
• Provide resistance to viral infections in nearby cells
• Inhibit the ability of a virus to replicate and assemble new virus particles

Adaptive Immunity (Specific)

• Acquired ability to defend against specific pathogens by prior exposure
• Also known as the learned response
• Systemic: Not restricted to a local infection site
• Has memory: Recognizes and mounts a stronger attack on previously encountered pathogens

Two Types of Adaptive Immunity

• Cell-mediated: Uses T cells to present antigens to killer cells
  • Attacks infected host cells (viruses, fungi), cancer cells, and foreign cells
• Antibody-mediated: Uses B cells to make antibodies to tag antigens for death
  • Attacks bacteria and viruses

Life of a B Cell

• Begins as a stem cell in bone marrow
• Maturation/stays in bone marrow
• Mature B cell (gains receptors)
• Enters the lymph and blood (humoral)
• Exposed to antigen
• Activated B cell
  • Plasma cells: Make antibodies
  • Memory cells: For second exposure

B Lymphocytes

• B cells have receptors to identify foreign antigens, which are usually proteins from foreign viruses, bacteria, and allergens
• After recognizing the matching antigen, B cells undergo division forming a clone
• B cells develop into plasma cells or memory cells

Antibodies

• AKA immunoglobulins (proteins)
• Part of the gamma globulin class of plasma proteins
• Have a similar basic structure but differences provide for antibody specificity

Complement

• Complements the action of antibodies
• Activated by pathogens (nonspecific) or exposure to antibodies (specific)
• When activated, proteins build a pore in the foreign cell, which is called the membrane attack complex (MAC)
• Osmotic influx of water, cell swells, and bursts

B Cells and T Cells

• Stem cell → Antigen → B lymphocyte/T lymphocyte
• B cells produce antibodies, while cytotoxic T cells destroy infected cells
• T cells involved need help from helper T cells which activate the B and cytotoxic T cells

Life of a T Cell

• Begins as a stem cell in bone marrow.
• Travels to the thymus.
• Develops into a mature T cell with receptors.
• Enters lymph and blood.
• Exposed to an antigen
• Develops into a sensitized T cell:
• Helper T cell
• Cytotoxic T cell
• Suppressor T cell

T Lymphocytes

• Cell-mediated immunity
• Mature in thymus
• Activated by antigens presented by antigen-presenting cells
  • Examples: macrophages, dendritic cells, B lymphocytes
• T cells activated promote action of macrophages and lymphocytes

Types of T Cells

• Helper T cell (CD4):
  • 60-80% of T cells
  • Secretes proteins to assist other systems
  • B cell growth factor
  • Interleukins for T cell growth
• Suppressor/regulatory T cell:
  • Prevents overactive immune responses

Cytotoxic/Killer T Cell (CD8)

• Releases perforin to create a pore in foreign cells
• Causes osmosis and destruction of the infected cell
• Involved in transplant rejection, defending against viruses, and fungi

MHC Molecules

• Occur in lymph nodes
• Found on all cells
• Signals infected cells

Interaction between Class-2 MHC & Helper T cell

• Occurs in lymph nodes
• Macrophage presents antigen to cells

Primary and Secondary Responses

• Primary response (1st exposure): Latency of 5-10 days before specific antibodies are made
• Antibody levels plateau, decrease after a few weeks
• Secondary response (Subsequent exposure): Antibody production is more rapid and sustained

Self-Antigens

• Human cells have surface proteins (self-antigens)
• Immune cells typically do not attack self-proteins
• Foreign cell proteins can trigger an immune response. This aspect is important in organ transplant rejection.

T cells recognize MHC

• The pathogen is taken in by a phagocyte
• A protein is put into a membrane called MHC
• MHC molecules on cell membranes come in different combinations.
• T cells recognize MHC plus antigen
• Activates specific T cell response to specific antigens

Questions of the Day

• Explain how antibodies help destroy invading bacterial cells.
• Describe the interaction between the antigen-presenting cell and T cell
• Compare and contrast B and T lymphocytes

Autoimmune Diseases

• Failure of the immune system to tolerate self-antigens
• Formation of autoreactive T cells and B cells.
• Production of autoantibodies (e.g., Myasthenia Gravis, antibodies to nicotinic cholinergic receptors on skeletal muscle).

Test your understanding of innate and adaptive immunity in this quiz. Compare their roles, particularly the functions of T cells and B cells. Additionally, explore key concepts such as local inflammation and the process of phagocytosis.