Adaptive Immune System Notes part

Adaptive Immune System

• Activated slower than the innate immune system but lasts longer.

• Involves lymphocytes (B and T cells).

B Cells

• Main function: produce and secrete antibodies.

• B cells turn into plasma cells (antibody-secreting cells).

T Cells

• Become effector T cells.

• Two main types:

  • Helper T cells: Help activate other immune cells.

  • Cytotoxic T cells: Kill infected cells.

Importance of Adaptive Immunity

• Raises an immune response against pathogens that evade the innate immune system.

• Needed to completely clear an infection when innate immunity isn't enough.

Memory

• Key feature differentiating it from the innate immune system.

• Adaptive immune system adapts and remembers pathogens.

• Repeat exposures lead to a faster, larger, and more effective response.

• First exposure: Takes about 10 days for antibody levels to increase.

• Second exposure: Antibody levels increase much earlier (around day three), and the response is faster and larger.

• Lymphocytes remember the pathogen.

• Vaccinations: Expose you to an antigen, priming your immune response, leading to memory against that antigen.

• B and T cells are pre-exposed in a non-infectious state.

• When encountering the actual pathogen, the adaptive immune system recognizes it and mounts a quick response.

Specificity and Diversity

• B and T cells express antigen-binding receptors on their surface.

• Each B and T cell has approximately 1000 receptors, all specific to a single antigen.

• A single B or T cell can only recognize one type of antigen from one type of pathogen (e.g., one B cell specific to COVID, another to influenza A).

• Different B and T cells have different antigen-binding receptors to detect different antigens, providing diversity.

Non-Reactivity to Self

• Adaptive immune system generally doesn't attack the body's own cells.

• Failure of this leads to autoimmune diseases.

Humoral vs. Cell-Mediated Immunity

• Two subdivisions of the adaptive immune system, differing by the type of lymphocyte involved (B cells or T cells).

Humoral Immunity

• Involves B cells producing antibodies.

• Antibodies prevent extracellular pathogens from entering and infecting cells.

• Eliminates pathogens before they enter cells.

Cell-Mediated Immunity

• Involves T cells.

• Focused on intracellular microbes.

• Important when pathogens have already entered cells and are actively infecting them.

• Helper T cells: Activate phagocytes to destroy pathogens and activate B cells to produce antibodies.

• Cytotoxic T cells: Kill infected cells.

• Humoral immunity (B cells) prevents pathogens from entering cells (extracellular response).

• Cell-mediated immunity (T cells) acts on pathogens that have already entered cells.

Humoral Immunity - B Cells

• Involves B cells secreting antibodies.

• Naive B cells: B cells that haven't encountered an antigen yet, express antigen-binding receptors (immunoglobulins) on their plasma membrane.

• When an antigen binds to the specific B cell receptor, the B cell is activated and undergoes clonal expansion (proliferation).

• Helper T cells assist in activating B cells.

• After clonal expansion, B cells differentiate into:

  • Plasma cells: Synthesize and secrete antibodies specific to the antigen. Short-lived (4-7 days) but produce many antibodies (approximately 2,000 antibody molecules every second).

  • Memory B cells: Long-lived cells responsible for the faster response upon re-encountering the antigen; they circulate and monitor for future infections.

Antibody Classes

• Five different classes of antibodies, each with different features and functions:

  • IgM: Most common class produced in response to an antigen.

  • IgG: Can cross the placenta during pregnancy, providing immunity to the fetus.

  • IgE: Responsible for allergies (e.g., hay fever).

Cell-Mediated Immunity - T Cells

• Controlled by T cells.

• T cells recognize intracellular antigens.

• Antigen must be presented to the T cell by an antigen-presenting cell (APC) using a major histocompatibility complex (MHC) molecule.

• APCs: Macrophages and dendritic cells.

• Infected cells present the antigen to the T cell via the MHC molecule.

• Antigen binds to the MHC molecule, and the complex is transported to the cell surface.

• T cells only detect intracellular antigens presented by APCs.

MHC Classes

• Two classes of MHC molecules:

  • MHC class one: Expressed on all nucleated cells (not specific to immune cells).

  • MHC class two: Expressed only by APCs (dendritic cells, macrophages, and B cells).

• APCs are found scattered throughout the body, especially in areas where pathogens can easily enter (skin, GI tract, respiratory tract).

• They reside in tissues, waiting for an antigen.

• Upon detecting an antigen, they are activated and migrate to the spleen or lymph nodes.

• In the lymph nodes, they present the antigen to T cells, activating them.

Example: Pathogen in the Skin

• Pathogen enters through a cut.

• Langerhans cells (macrophages) in the epidermis pick up the antigen.

• Langerhans cells migrate to the dermis and present the antigen to dendritic cells.

• Dendritic cells are activated and migrate to the lymph nodes.

• In the lymph node, dendritic cells mature and increase MHC expression.

• They present the antigen to naive T cells, activating them.

Function of MHC Classes

• MHC class one activates cytotoxic T cells to kill infected cells.

• MHC class two activates helper T cells to help other immune cells.

• Macrophages presenting antigen via MHC class two activate helper T cells, which then activate the macrophage to destroy the pathogen.

• B cells can also activate T cells by presenting an antigen via MHC class two; the activated helper T cell then helps activate the B cell to produce more antibodies.

T Cell Activation

• Naive T cells: T cells that haven't encountered an antigen.

• When an APC presents an antigen to a T cell, a sequence of events occurs, and the T cell is activated and undergoes clonal expansion.

• T cells differentiate into:

  • Effector T cells: Respond to clear the infection.

  • Memory T cells: Reside in the tissue at the site of infection and act as a surveillance mechanism for long-lived immunity.

• Memory B and T cells (lasting months or years) help activate humoral and cell-mediated immunity upon re-encountering a pathogen.