lesson 22.3

Key Players in T Cell Activation:
- Cytotoxic T Cells:
- Recognize antigens presented by MHC class I.
- Directly attack and destroy infected or abnormal cells.
- Helper T Cells:
- Recognize antigens presented by MHC class II.
- Stimulate the immune response by releasing cytokines.
- Regulatory T Cells:
- Inhibit immune responses to prevent autoimmunity.
- Memory T Cells:
- Remain active for a faster response upon secondary exposure.
Mechanism of T Cell Response:
- Antigen presentation by MHC Class I:
- Indicates the presence of infected cells.
- Activates cytotoxic T cells.
- Cytotoxic T Cells:
- Lyse infected cells upon activation.
- Can be inhibited by regulatory T cells releasing cytokines.
- Proliferation and differentiation lead to memory T cells upon re-exposure.

Function of B Cells:
- Part of the adaptive immune response.
- Utilize B cell receptors (BCR), which are antibodies in the precursor state.
Sensitization and Activation Process of B Cells:
- Sensitization:
- Requires binding to free antigens (not inside infected cells).
- Free antigens in interstitial fluid bind to BCR and are internalized via endocytosis.
- Activation:
- Processed antigens are re-presented on MHC class II molecules.
- Helper T cells recognize presented antigens and release cytokines, propelling B cell activation.
- Activated B cells undergo division and differentiation into plasma cells and memory B cells.

What are Antibodies?
- Antibodies are soluble proteins produced by activated B cells (plasma cells).
Antibody Structure:
- Composed of heavy chains (purple) and light chains (blue).
- Distinction between constant region (same across antibody types) and variable region (specific to the antigen).
- Antigen Binding Site:
- Located in the variable region, highly specific for a particular antigen.
Antigen-Antibody Complex:
- Formation occurs upon binding of an antibody to an antigen.
- Binding leads to different outcomes:
- 1. Neutralization:
  - Immobilizes pathogens to prevent them from attacking cells.
- 2. Agglutination:
  - Clumping of pathogens to facilitate their removal.
- 3. Activation of Complement System:
  - Series of proteins leading to pathogen destruction.
- 4. Phagocyte Attraction:
  - Attracts immune cells (e.g., macrophages) to eliminate pathogens.
- 5. Recruitment of Mast Cells:
  - Leads to inflammation, enhancing the immune response.

Different Classes of Antibodies:
- IgG:
- Major class (80% of antibodies), involved in the destruction of pathogens.
- Includes anti-Rh antibodies important for blood types.
- IgE:
- Involved in inflammatory responses, particularly allergies.
- IgD:
- Acts as a B cell receptor for sensitization; not directly involved in eliminating pathogens.
- IgM:
- First antibody secreted upon B cell activation, involved in blood typing (anti-A and anti-B).
- Pentamer structure (five antibodies).
- IgA:
- Found in glandular secretions (mucosal surfaces).

Primary vs. Secondary Immune Response:
- Primary Response:
- First exposure to an antigen, longer response time.
- Initially produces IgM, then class switches to IgG.
- Secondary Response:
- Rapid response upon re-exposure; memory cells respond immediately.
- Predominantly IgG antibody production.

Types of Hypersensitivity:
- Type I:
- Immediate hypersensitivity (allergies), mediated by IgE.
- Can lead to anaphylaxis in severe cases.
- Type II:
- Cytotoxic reactions mediated by IgG or IgM against cell-bound antigens.
- Example: Blood transfusion mismatches.
- Type III:
- Immune complex disorders, mediated by IgG or IgM against soluble antigens.
- Examples: rheumatoid arthritis, lupus.
- Type IV:
- Delayed hypersensitivity; involves T cells.
Immunodeficiency Diseases:
- Result from genetic mutations affecting B cells, T cells, or natural killer cells (e.g., SCID).
- Can also result from infections (e.g., HIV leading to AIDS).