Antibody Structure and Function

Overview

• Antibody Structure
  • Antibodies have distinct regions: FC fragment and Fab fragment.
  • Important for USMLE and COMLEX exams due to the various parts and functions.

Regions of the Antibody

FC Fragment

• Meaning
  • FC stands for "crystallizable," which is largely irrelevant for exams.
  • C-words associated with FC:
    • Constant: Does not change.
    • Complement: Involved in complement cascade.
    • Cells bind here: Macroasia and mast cells can bind.
    • Controlling the isotype: Determines antibody class (e.g., IgM, IgG).
• Characteristics
  • Contains only heavy chains.

Fab Fragment

• Meaning
  • AB stands for "antigen binding."
  • It is the region with the antigen binding sites.
• Characteristics
  • Contains both heavy and light chains.
  • Variable Region: Genetic composition can change to recognize various antigens.
  • Responsible for determining the "idiotype."

Hinge Region

• Connects Fab and FC fragments.
• Allows the antibody to assume different shapes, enhancing immune function.
• Disulfide bonds present, connecting heavy and light chains, and hinge region.

Key Concepts

• Isotype vs. Idiotype
  • Isotype: Type of antibody (e.g., IgM, IgG); controlled by constant regions.
  • Idiotype: Ability to bind different antigens; determined by variable regions.
• Antigen Binding Sites
  • Located at the tips of the Fab fragment.
  • Variable for diverse antigen recognition.

Exam Tips

• Identifying Regions: Be familiar with identifying regions responsible for isotype and idiotype.
• High Yield Questions
  • Determine isotype: Look at constant heavy regions (1, 2, and 3 are most accurate).
  • Differentiate between contributions of FC and Fab fragments.

Important Points

• Understand the distinction between light and heavy chains.
• Recognize the role of disulfide bonds in structure.
• Familiarize with constant vs. variable regions and their functions.

Next Topics

• Differences between various antibody isotypes (e.g., IgA, IgG, IgM).