Adaptive Immunity Characteristics

• Adaptive immunity is also known as acquired immunity.
• Adaptive immunity can recognize, remember, and mount highly targeted responses against specific antigens.
• Adaptive immunity is crucial for long-term protection and plays a crucial role in defending the body against infections.

Characteristics of Adaptive Immunity

• Lag period/latency period refers to the initial phase of the immune response following the first exposure to a pathogen.
• During the lag period, the immune system recognizes the pathogen, activates specific immune cells, and starts to generate a tailored response.
• Delayed activation characterizes the lag period, and it can last several days to a week, depending on the pathogen or individual factors.
• Memory is the immune system's ability to "remember" specific pathogens or antigens encountered previously.
• Antigens are substances that stimulate an immune response.
• Foreign antigens are introduced from outside the body.
• Self-antigens are molecules produced by body cells to identify them as "self" or part of the body.
• The body's response to self-antigens is compromised if a person has autoimmune diseases.
• Autoimmune diseases result when self-antigens stimulate the destruction of normal tissues.
• Examples of autoimmune diseases are rheumatoid arthritis, multiple sclerosis, and systemic lupus erythematosus.

Memory Formation in Adaptive Immunity

• Memory B cells persist in the body for an extended period once generated.
• When the body encounters the same pathogen again, memory B cells quickly differentiate into plasma cells, which produce large quantities of antibodies.
• Memory T cells, including both cytotoxic T cells and helper T cells, are generated during the primary immune response.
• Upon re-infection, memory T cells can quickly recognize and destroy infected cells or activate other immune cells, contributing to a faster and more targeted immune response.
• The presence of immunological memory is the foundation for the development and effectiveness of vaccines.
• Specificity is the immune system's ability to recognize and respond to specific pathogens or antigens with a high degree of precision.
• Specificity is achieved through the diverse range of receptors on lymphocytes, particularly T cells and B cells.

Specificity in Adaptive Immunity

• T cells express T cell receptors (TCRs) on their surface.
• Each T cell has a unique TCR that can specifically recognize and bind to a particular antigen presented by major histocompatibility complex (MHC) molecules.
• This specific interaction allows T cells to distinguish between different pathogens and respond selectively to the ones they are equipped to combat.
• B cells produce antibodies, and each B cell has surface antibodies that recognize specific antigens.
• The antigen-binding region of antibodies, known as the variable region, is highly diverse among different B cells.
• This diversity allows antibodies to bind specifically to a wide range of antigens.

Ways to Acquire Adaptive Immunity

• Active immunity: Immunity is provided by the individual's own immune system.
• Natural: Antigens are introduced through natural exposure.
• Artificial: Antigens are deliberately introduced in a vaccine.
• Passive Immunity: Immunity is transferred from another person or an animal.
• Natural: Antibodies from the mother are transferred to her child across the placenta or in milk.
• Artificial: Antibodies produced by another person or an animal are injected.

Branches of Adaptive Immunity

• Humoral Immunity involves the action of B cells and antibodies (immunoglobulins) to defend the body against pathogens.
• Humoral Immunity is effective against extracellular pathogens, such as bacteria and toxins, which circulate in the body fluids (blood & lymph).

Functions of Humoral Immunity

• Neutralization: Antibodies can neutralize pathogens by binding to them and preventing them from entering or infecting host cells.
• Opsonization is a process where pathogens, such as bacteria or other foreign particles, are marked for destruction by phagocytic cells.
  • Opsonization is facilitated by the binding of opsonins to receptors on the surface of phagocytes.
• Opsonins are molecules that enhance the process of phagocytosis by binding to the surface of the pathogen or particle, effectively "marking" it for easier recognition and uptake by phagocytes. : Examples of opsonins are antibodies IgG and IgM and complement proteins.
• Complement Activation: Antibodies can trigger the complement system, a group of proteins that contribute to the immune response.
• Complement activation leads to the formation of membrane attack complexes that can lyse pathogens or mark them for phagocytosis.
• Agglutination and Precipitation: Antibodies can cause pathogens to clump together (agglutination) or form insoluble complexes (precipitation), making it easier for phagocytes to engulf and eliminate them.
• Immunologic Memory: B cells, upon encountering a specific antigen, can differentiate into memory B cells.
  • Memory B cells "remember" the antigen and enable a quicker and more robust immune response upon re-exposure to the same pathogen.

Key Components of Humoral Immunity

• B lymphocytes are a type of white blood cell that plays a central role in humoral immunity.
• Each B cell expresses unique surface antibodies that are specific to a particular antigen.
• These antibodies serve as receptors for recognizing and binding to antigens.
• Antibodies (Immunoglobulins) are proteins produced by B cells (plasma cells) in response to the presence of specific antigens.
• Each antibody is specific to a particular antigen, and the diversity of antibodies allows the immune system to recognize a wide range of pathogens.
• Cell-mediated Immunity involves the activation of specific immune cells, particularly T lymphocytes (T cells), to directly attack and eliminate infected or abnormal cells.
• Cell-mediated Immunity is crucial for defending the body against intracellular pathogens, such as viruses, as well as eliminating cancerous cells.

Functions of Cell-Mediated Immunity

• Defense against intracellular pathogens, including viruses and certain bacteria, and elimination of cancerous cells.
• Activation of macrophages.
• Coordination and regulation of immune responses.
• Rejection of transplanted tissues (graft rejection) due to the recognition of foreign antigens.

Key Components of Cell-Mediated Immunity

• T lymphocytes are the central players in cell-mediated immunity.
• These cells mature in the thymus, hence the designation "T cells.".

Subtypes of T cells

• Helper T Cells (CD4+) coordinate immune responses by activating other immune cells.
• Helper T Cells interact with antigen-presenting cells (APCs) and assist in activating B cells and cytotoxic T cells.
• Cytotoxic T Cells (CD8+) directly target and kill infected or abnormal cells.
• Antigen-Presenting Cells (APCs), such as dendritic cells, macrophages, and B cells, play a crucial role in cell-mediated immunity.
• APCs capture, process, and present antigens derived from pathogens to T cells.
• This presentation occurs through major histocompatibility complex (MHC) molecules.

Key stages of the cell-mediated immune response

• Antigen Presentation: APCs present foreign antigens to T cells via MHC molecules.
• T Cell Activation: T cells recognize the antigen and become activated.
• Clonal Expansion: Activated T cells proliferate, creating effector and memory cells.
• Differentiation: T cells differentiate into specialized subsets (e.g., Th1, Th2, Th17, Tregs, CD8+).
• Effector Phase: Effector T cells (CD8+ and CD4+) carry out immune functions, such as killing infected cells and activating other immune cells.
• Memory Formation: Memory T cells remain in the body for long-term immunity.