Adaptive Immune System

Overview of Adaptive Defenses

The adaptive (specific) immune system is a highly specialized defense mechanism that protects the body against infectious agents and abnormal cells. Unlike the innate immune system, which provides general protection, the adaptive system learns to recognize and target specific antigens.

• Specificity: Targets particular antigens associated with pathogens or abnormal cells.

• Memory: Remembers previous encounters, allowing for faster and stronger responses upon re-exposure.

• Delayed Response: Activation requires initial exposure and takes time to develop.

• Overlapping Arms: Includes humoral (antibody-mediated) and cellular (cell-mediated) immunity.

Humoral versus Cellular Immunity

Comparison and Mechanisms

Adaptive immunity is divided into two main types: humoral and cellular. Each targets pathogens in different locations and uses distinct mechanisms.

• Humoral Immunity: Mediated by B lymphocytes and antibodies circulating in body fluids (blood, lymph, interstitial fluid).

• Cellular Immunity: Mediated by T lymphocytes that directly attack infected or abnormal cells.

Key Differences:

• Humoral: Effective against pathogens outside cells (e.g., bacteria in blood).

• Cellular: Effective against pathogens inside cells (e.g., viruses within host cells).

Example: If a bacterium is in the bloodstream, humoral immunity uses antibodies to tag it for destruction. If a virus is inside a cell, cellular immunity uses T cells to destroy the infected cell.

Antigens

Definition and Properties

Antigens are substances that can mobilize adaptive defenses and provoke an immune response. They are typically large, complex molecules not normally found in the body.

• Immunogenicity: The ability of an antigen to stimulate proliferation of specific lymphocytes and antibody production.

• Reactivity: The ability of an antigen to react with products of activated lymphocytes and antibodies.

• Examples: Foreign proteins, polysaccharides, lipids, and nucleic acids.

Complete Antigens: Possess both immunogenicity and reactivity.

Haptens (Incomplete Antigens): Small molecules (e.g., peptides, nucleotides, hormones) that are not immunogenic by themselves but can become immunogenic when attached to body proteins. Common sources include poison ivy, animal dander, detergents, and cosmetics.

Self-Antigens: MHC Proteins

Major Histocompatibility Complex (MHC)

MHC proteins are self-antigens found on the surface of cells. They play a crucial role in immune recognition and are unique to each individual.

• Class I MHC: Found on virtually all body cells; display self-antigens or fragments of foreign antigens.

• Class II MHC: Found on certain immune cells (e.g., antigen-presenting cells); display fragments of foreign antigens to T cells.

Function: MHC proteins present antigens to T cells, helping the immune system distinguish self from non-self.

Cells of the Adaptive Immune System

B Lymphocytes (B Cells)

B cells are responsible for humoral immunity. They mature in the bone marrow and produce antibodies that bind to specific antigens.

• Plasma Cells: Effector B cells that secrete antibodies.

• Memory B Cells: Provide immunological memory for rapid response upon re-exposure.

T Lymphocytes (T Cells)

T cells are responsible for cellular immunity. They mature in the thymus and directly attack infected or abnormal cells.

• Helper T Cells: Activate B cells and other immune cells.

• Cytotoxic T Cells: Destroy virus-infected or cancerous cells.

Antigen-Presenting Cells (APCs)

APCs (e.g., dendritic cells, macrophages) ingest pathogens and present antigen fragments to T cells, initiating the adaptive immune response.

• Dendritic Cells: Bridge innate and adaptive immunity; found in lymphoid organs and connective tissues.

• Macrophages: Phagocytize pathogens and present antigens to T cells.

Summary Table: Humoral vs Cellular Immunity

Additional info: The notes use analogies (security guard vs. wanted poster) to illustrate the difference between innate and adaptive immunity, emphasizing the specificity and learning capacity of the adaptive system.