Helper T-Cells and Immune Response

• The immune system protects the body from pathogens.
• Key concepts include pathogens, antigens, antigen-presenting cells, cytokines, phagocytosis, and apoptosis.

Helper T-Cells

• Helper T-cells can only recognize antigens presented on a Major Histocompatibility Complex (MHC).
• The CD3 complex on the T-cell membrane is essential for antigen recognition.

CD4 Molecule

• The CD4 molecule is located next to the CD3 complex.
• CD4 binds to the MHC molecule, securing the interaction between the T-cell receptor and the MHC molecule.

CD28 Molecule

• CD28 is a structure on the surface of the T-cell that binds to B7.
• B7 is a structure on the antigen-presenting cell.

Interaction with Bacteria

• Bacteria invade tissue.
• Helper T-cells arrive at the site of infection.
• Helper T-cells release cytokines.
• Cytokines attract neutrophils to the area of infection.
• Neutrophils perform phagocytosis to engulf and destroy pathogens.
• Cytoplasmic granules within neutrophils contain phagolysosomes for digestion, and an oxidative burst with reactive oxygen species aids in killing pathogens.

Cytotoxic T-Cells

• Cytotoxic T-cells target virus-infected cells by recognizing viral antigens presented on MHC I molecules.
• Upon attaching to the virus's antigen in the MHC I molecule, the cytotoxic T-cell releases granules containing perforin and granzymes.
• Perforin punches holes in the lipid bilayer of the target cell.
• Granzymes trigger apoptosis (programmed cell death) in the target cell.

Natural Killer Cells

• Natural Killer (NK) cells also contain granzymes and perforin granules.
• Many viruses remove MHC I from the host cell's membrane to evade cytotoxic T-cells.
• The absence of MHC I triggers NK cells to attack the cell.
• NK cells use perforin to create holes in the target cell membrane and granzymes to induce apoptosis.

B-Cells

• B-cells function as antigen-presenting cells via MHC II.
• They present antigens (on MHC class II molecules) to CD4+ T-cells.
• When the CD40 molecule on the B-cell binds to the CD40L molecule on the Helper T-cell, the B-cell expresses a cytokine receptor, prompting the T-cell to secrete cytokines.
• B-cells can bind to antigens directly without needing MHC molecules.
• They internalize pathogens and load antigens onto an MHC II Molecule.
• When CD40 and CD40L combine, the B-cell gets triggered to express a cytokine receptor, and the T-cell is triggered to secrete cytokines.
• Cytokines trigger the B-cell to transform into a plasma cell and secrete antibodies specific to the pathogen.
• Antibodies circulate in the plasma to attach to specific pathogens ("tags") for destruction by other immune system cells.

Dendritic Cells

• Dendritic cells phagocytose pathogens and present antigens to T-cells.
• They reside in tissues in contact with the external environment, such as skin and the gastrointestinal tract.
• Dendritic cells travel to lymph nodes to interact with T-cells.
• One role of dendritic cells is to present an antigen to a T-cell and activate it.
• Activation requires CD28 on the T-cell to bind with B7 on the dendritic cell, and the T-cell to bind to the antigen on the MHC complex.