• Describe how the immune system distinguishes between self and non-self.
• Explain how antigen presentation initiates immune responses.
• Identify different types of pathogens and allergens.

Introduction

For an immune response to begin, the body must first detect the presence of foreign antigens. This process starts with the immune system recognising whether a molecule is part of the body (self) or comes from an invader (non-self). Once detected, the foreign antigen is processed and displayed to cells of the adaptive immune system, which then coordinate a targeted response.

Key Concepts

Antigens

• Unique molecules, often proteins or polysaccharides, that can trigger an immune response.
• Found on the surface of pathogens, transplanted tissues, or in toxins.

Self-antigens vs Non-self antigens

• Self-antigens: Molecules found naturally on the body’s own cells (e.g., MHC I markers).
• Non-self antigens: Molecules from pathogens, foreign substances, or abnormal cells that are recognised as threats.

MHC Markers (Major Histocompatibility Complex)

• MHC I: Found on the surface of all nucleated cells. They help immune cells identify normal self-cells and can also display fragments of intracellular pathogens (e.g., viruses) to cytotoxic T cells.
• MHC II: Found only on specialised antigen-presenting cells (APCs). They display fragments of extracellular pathogens to helper T cells, triggering the adaptive immune response.

Antigen-Presenting Cells (APCs)

• Include dendritic cells, macrophages, and B cells.
• Their role is to capture pathogens, break them down, and present antigen fragments on MHC II to activate helper T cells, which then coordinate immune defences.

Pathogen Types

• Cellular pathogens: Living organisms such as bacteria, protozoa, fungi, and parasitic worms.
• Non-cellular pathogens: Viruses (which must infect host cells to replicate) and prions (infectious misfolded proteins).

Allergens

• Usually harmless antigens (e.g., pollen, dust mite proteins) that cause an exaggerated immune response in some individuals.
• This response often involves the production of IgE antibodies, mast cell activation, and histamine release, leading to symptoms such as itching, swelling, and airway constriction.