Understanding the Immune System

• The immune system protects the body from pathogens like bacteria, viruses, fungi, and parasites.
• Organs, cells, and molecules coordinate immune responses to protect the body.

Two Primary Branches of the Immune System:

• The innate immune system is fast, non-specific, and has no memory.
• The adaptive immune system is specific, slower, and memory-based.

Immune System Components:

• White Blood Cells (Leukocytes) are key components of the immune system.
• Myeloid cells include neutrophils, basophils, eosinophils, and monocytes (macrophages, dendritic cells).
• Lymphocytes include T cells, B cells, and Natural Killer (NK) cells.

Innate Immune Response:

• It serves as the first line of defense and is rapid and non-specific.
• It recognizes invaders through pattern recognition receptors (PRRs).
• It responds the same way to repeated infections with no memory.

Cells of the Innate Immune System:

• Neutrophils are first responders that phagocytose pathogens.
• Neutrophils destroy pathogens through phagolysosome formation and oxidative burst.
• Macrophages phagocytose pathogens.
• Macrophages release cytokines to signal other immune cells.
• Macrophages present antigens to activate T cells.
• Dendritic cells are found in skin and mucosal tissues.
• Dendritic cells phagocytose pathogens, process antigens, and migrate to lymph nodes.
• Dendritic cells present antigens to T cells, linking to the adaptive immune response.
• Natural Killer (NK) Cells target virus-infected cells and cancer cells.
• Natural Killer (NK) Cells kill through perforin and granzymes, leading to apoptosis.
• Natural Killer (NK) Cells recognize the absence of MHC I molecules on infected cells.

Adaptive Immune Response:

• It is slower but highly specific.
• It generates memory cells for stronger future responses.

B Cells & Antibody-Mediated Immunity:

• B cells mature in the bone marrow.
• When activated, B cells become plasma cells that produce antibodies.
• Antibodies function by neutralizing pathogens, marking pathogens for destruction (opsonization), and activating the complement system.

T Cells & Cell-Mediated Immunity:

• T-Cell Receptor (TCR) binds antigen presented on MHC molecules, initiating T-cell activation.
• CD4 Helper T Cells bind MHC II which then activates immune responses.
• CD8 Cytotoxic T Cells bind MHC I, resulting in the killing of infected cells.

Helper T Cells (CD4+):

• They support immune functions by secreting cytokines.
• They activate B cells, macrophages, and cytotoxic T cells.
• They recognize MHC II molecules from antigen-presenting cells (APCs).

Cytotoxic T Cells (CD8+):

• They kill infected cells by binding MHC I molecules.
• They release perforin (punches holes) and granzymes (triggers apoptosis).

Major Histocompatibility Complex (MHC) Molecules:

• MHC I molecules are found on all nucleated cells.
• MHC I molecules present antigens to Cytotoxic T Cells (CD8+).
• MHC I molecules signal infected or cancerous cells for destruction.
• MHC II molecules are found on antigen-presenting cells (APCs) like macrophages, dendritic cells, and B cells.
• MHC II molecules present antigens to Helper T Cells (CD4+).
• MHC II molecules activate helper T cells to coordinate immune responses.

T Cell Activation & Clonal Expansion:

• Antigen Recognition requires TCR binding to antigen on the MHC molecule.
• Co-stimulation occurs when CD28 on the T cell binds B7 on APC.
• Without co-stimulation, the T cell enters anergy (inactive state).
• Activated Helper T Cells secrete IL-2.
• IL-2 binds IL-2 receptors on activated T cells causing rapid proliferation.

Immunological Memory:

• After infection, memory T and B cells persist.
• Memory cells provide faster and stronger responses upon re-exposure.
• Vaccines create memory cells without causing disease.

Immune Response to Pathogens:

• Macrophages & neutrophils phagocytose bacteria during bacterial infections.
• Dendritic cells present antigens to T cells during bacterial infections.
• CD4+ Helper T cells activate B cells, leading plasma cells to secrete antibodies during bacterial infections.
• Antibodies opsonize bacteria for destruction during bacterial infections.
• Infected cells display viral antigens on MHC I in response to viral infections.
• Cytotoxic T cells (CD8+) kill infected cells during viral infections.
• Natural Killer cells attack cells lacking MHC I during viral infections.
• Antibodies neutralize extracellular viruses during viral infections.

Summary of Key Immune Cells & Functions:

• Neutrophils perform phagocytosis and oxidative burst and target bacteria.
• Macrophages perform phagocytosis and cytokine secretion and target bacteria and fungi.
• Dendritic Cells are responsible for antigen presentation to T cells and target various pathogens.
• Natural Killer Cells kill virus-infected and cancer cells and target infected cells.
• Helper T Cells (CD4+) activate immune cells and target antigen-presenting cells.
• Cytotoxic T Cells (CD8+) kill infected cells and target cells with MHC I.
• B Cells produce antibodies and target pathogens in the blood.