Biochemistry of the Immune Response

Biochemistry of the Immune Response

Introduction

The immune response is a complex biological process that protects the body from infection and disease. It involves the activation of various cells and molecules that work together to identify and eliminate foreign invaders. This intricate system relies on a series of biochemical interactions to effectively combat pathogens and maintain homeostasis.

Key Components

• Immunoglobulins (Antibodies): Glycoproteins produced by plasma cells (differentiated B cells) that bind to specific antigens, neutralizing their activity through various mechanisms such as opsonization, complement activation, and neutralization of toxins.
• Cytokines: Signaling proteins (e.g., interleukins, interferons, TNF-α) that regulate the immune response by coordinating cell-cell communication, mediating inflammation, and influencing cell growth and differentiation. Their specific actions vary greatly depending on the cytokine and the target cell.
• Complement System: A cascade of serum proteins that amplify the inflammatory response, directly lyse pathogens (membrane attack complex), and enhance phagocytosis (opsonization).
• Phagocytic Cells: Specialized cells (e.g., macrophages, neutrophils, dendritic cells) that engulf and destroy foreign particles through phagocytosis. They also play a crucial role in antigen presentation.
• Antigen-Presenting Cells (APCs): Cells (e.g., dendritic cells, macrophages, B cells) that process foreign antigens and present them to T cells via MHC molecules, initiating the adaptive immune response.
• Major Histocompatibility Complex (MHC) Molecules: Cell surface proteins that bind and present antigens to T cells. MHC class I presents intracellular antigens to cytotoxic T cells, while MHC class II presents extracellular antigens to helper T cells.
• T cells: Lymphocytes that play a central role in cell-mediated immunity. Helper T cells (CD4+) coordinate the immune response, while cytotoxic T cells (CD8+) directly kill infected cells.
• B cells: Lymphocytes that produce antibodies and contribute to humoral immunity. They also act as APCs.

Process

When the immune system encounters a foreign invader (antigen), it triggers a cascade of biochemical events:

1. Antigen recognition: Pattern recognition receptors (PRRs) on innate immune cells recognize pathogen-associated molecular patterns (PAMPs), while T and B cells recognize specific antigens via their receptors.
2. Antigen presentation: APCs process and present antigens to T cells.
3. T cell activation: T cell receptors (TCRs) bind to MHC-antigen complexes, triggering T cell activation and proliferation.
4. B cell activation and antibody production: Helper T cells activate B cells, leading to antibody production and the formation of memory B cells.
5. Complement activation and phagocytosis: Antibodies and complement proteins opsonize pathogens, facilitating their phagocytosis.
6. Cytokine release and inflammation: Cytokines mediate inflammation and recruit immune cells to the site of infection.
7. Elimination of pathogens: Pathogens are neutralized, killed, or phagocytosed.

Regulation

The immune response is tightly regulated to maintain homeostasis and prevent damage to the host. Regulatory T cells (Tregs) suppress immune activity to prevent excessive inflammation and autoimmune disorders. Other regulatory mechanisms include feedback loops involving cytokines and other signaling molecules.

Clinical Significance

Understanding the biochemistry of the immune response is crucial for developing therapies against infectious diseases, allergies, autoimmune disorders, and cancer. This knowledge is essential for vaccine development, immunotherapy, and organ transplantation.