AP Bio: The Immune System - Part 2

Chapter 43: Immune System Overview

Major Histocompatibility Complex (MHC)

• Role of MHCs:

  • Determine "self" from "non-self" in cells.

  • Present antigens by binding to proteins from pathogens (viruses or bacteria).

  • Critical for signaling to immune cells to initiate response.

Antigen-Presenting Cells (APCs)

• Common APCs:

  • Mainly macrophages, but can include other immune cells with MHCs.

  • MHC Class I molecules present on nearly all cells.

  • MHC Class II molecules primarily associated with immune cells (e.g., macrophages, T cells).

• Function:

  • Allow T helper cells and cytotoxic T cells to read antigens, triggering immune activation.

Immune Responses

• Types of Immune Responses:

  • Humoral Immunity:

    • Activates B cells leading to antibody production.

    • Effective against smaller pathogens: bacteria, viruses, and toxins.

  • Cell-Mediated Immunity:

    • Activates cytotoxic T cells to destroy infected or cancerous cells via perforin.

    • Targets larger pathogens: fungi, protozoans, and even larger parasites.

Roles and Functions of T Cells

• T Helper Cells:

  • Do not kill pathogens directly.

  • Use CD4 protein to recognize MHC molecules with antigens.

  • Secrete interleukin to activate other immune processes (both humoral and cell-mediated).

• Cytotoxic T Cells (TC):

  • Use CD8 protein to read MHC Class I on normal cells.

  • Directly responsible for killing infected cells through perforin.

B Cells and Antibody Response

• Activation of B Cells:

  • Often depends on T helper cell activation (T-dependent response).

  • Upon activation, B cells turn into plasma cells producing antibodies and memory cells for future responses.

• Structure of Antibodies:

  • Consist of variable regions specific to antigens (epitopes).

  • Types of antibodies vary in function (e.g., some cross placenta, some in breast milk).

Antibody Functions

• Neutralization:

  • Bind to viral attachment sites, preventing infection.

• Agglutination:

  • Clump antigens together, enhancing clearance by white blood cells.

• Precipitation:

  • Convert soluble toxins to insoluble forms for better clearance.

• Complement Activation:

  • Antibodies trigger a cascade of proteins that lead to cell lysis.

Immunity and Resistance Mechanisms

• Active Immunity:

  • Immune response from exposure to pathogens or vaccines.

  • Natural (getting sick) vs. Artificial (vaccines).

• Passive Immunity:

  • Gained through antibodies from another source (not generated by own immune response).

  • Natural (breast milk) vs. Artificial (injection of antibodies like rabies shots).

Blood Typing and Immune Response

• Blood Types:

  • A and B antigens; O type has neither, making it the universal donor.

  • Rh factor determines compatibility; negative can only receive from negative.

• Importance in Transfusions:

  • Mismatched blood types can trigger immune responses.

Immune System Dysfunctions

• Allergies:

  • Immune reaction to non-lethal antigens (pollen, peanuts).

  • Can cause anaphylactic shock due to inflammatory responses.

• Autoimmune Diseases:

  • Immune system mistakenly attacks own cells/tissues (e.g., diabetes, Crohn's disease).

• Immunodeficiency Diseases:

  • Conditions such as SCID or AIDS lead to compromised immune function.

  • Increased susceptibility to infections and diseases.