4- Immunology- Pt 2.docx

- **Antigen Receptors (T cells and B cells)** - T cells require antigen presentation (which involves the antigen coming to the T cell). - B cells are able to approach the antigen directly (meaning that the B cell can go to the antigen). - **Antigen Processing and P...

- **Antigen Receptors (T cells and B cells)** - T cells require antigen presentation (which involves the antigen coming to the T cell). - B cells are able to approach the antigen directly (meaning that the B cell can go to the antigen). - **Antigen Processing and Presentation (T cells)** - The MHC (major histocompatibility complex) molecule has 2 classes. - **Class 1 MHC** - CD8 (cytotoxic T cells) can only recognize antigens presented on MHC1 molecules. - Every nucleated cell has a class 1 MHC molecule. - MHC1 takes any proteins produced in the cells that are present in the cytosol, breaks them down into peptides, and displays these peptides on the cell surface. - CD8+ T cells is read as "CD8 positive T cells". - **Class 2 MHC** - CD4 (helper T cells) can only recognize antigens presented on MHC2 molecules. - MHC2 expressing cell types: Dendritic cells, macrophages, B cells. - **Naïve T Cell: Activation** - Naïve T cells are fully mature T cells that have not yet interacted with antigens. - They are located within lymph nodes. - The activation of naïve T cells are ALWAYS done by dendritic cells ONLY. - Proliferation: specific T cell clone - Differentiation: effector and memory T cells - Effectors: are those that can respond to and recognize antigens from any cells only AFTER dendritic cell activation. - **Naïve T cell: Activation Steps** - Step 1: Immature dendritic cells encounter antigens in the tissue. - Step 2: Migration to lymphoid tissue and maturation. - Step 3: Antigen presentation by mature dendritic cells in the lymph nodes. - **T Cell Subsets (CD4 and CD8)** - **CD4** is known as the **helper T cell**. - CD4 is involved in the activation of macrophages, B cells, and other cells. - CD4 has the following subsets/types: Th1, Th2, Th17 - **CD4+ Th1** cells are involved in cytokine secretion, activation of macrophages, and activation of CTLs (CD8+). - Th= T helper type - CD4 expresses surface molecules and secretes cytokines that activate other cells ("helping" other cells to respond to pathogens). - **CD8** is known as the **cytotoxic T cell**. - CD8 is involved in the killing of infected "target cells", and macrophage activation. - CD8 has the following subset/types: CTLs (cytotoxic T lymphocytes) - **CD8+ CTLs** will only kill pathogens, and will never produce cytokines. - CD8+ CTLs induce apoptosis of infected cells. - **Apoptosis** is when a cell dies without rupturing the cell membrane, thus preventing inflammation by keeping the pathogen contained within the cell during the killing of the pathogen. - In contrast, **lysis** allows the pathogen to leave the cell due to the rupturing of the cell membrane, causing inflammation. - **Adaptive Immunity: Humoral Response (B cells)** - B cells recognize antigens directly and present them to T cells. - Antigen recognition is not restricted by MHC presentation. - **B cell T-independent activation** - The production of IgM is the only possible response with T-independent activation. - T-independent activation does not produce memory cells. - T-independent activation B cells have low-affinity antibodies, and short-lived plasma cells. - **B cell activation by Th cells** - When T cells activate B cells, they can have any of the following responses: - Antibody secretion - Isotype switching - Affinity maturation - Memory B cell - B cell activation by Th cells induces a stronger response than T-independent activation. - T-dependent activation B cells have their isotype-switched, high-affinity antibodies, memory B cells, and long-lived plasma cells. - **Humoral responses involve:** - Immunoglobulins - (Ig) Isotypes or classes - Activates the most responses: IgG - Activates limited responses: IgM, IgA, IgE, IgD - **Ig Isotypes** - **Ig Isotypes have differences in:** - Heavy chain structure - Monomeric vs polymeric - Subclasses - Serum concentration - Half-life - Functions - **Ig Isotype distribution** - IgM: Blood - IgM is the major antibody of primary responses. - IgG: Blood and tissues - IgG is the major antibody of secondary responses. - IgA: Mucous Membranes - IgE: Epithelial tissue. - **Ig Isotype functions** - IgM: Activates classical pathway of complement - IgA: Mucosal immunity, neutralizing microbes and toxins within the GI and respiratory tract - IgE: Eosinophil and mast cell mediated defense against helminths - IgG: - Neutralizes microbes and toxins - Feedback inhibition of B cell activation - Neonatal immunity - Antibody dependent cellular cytotoxicity mediated NK cells - Opsonization of antigens for phagocytosis by macrophages and neutrophils