Immunology

Concept: Antibodies (immunoglobulins) are proteins produced by the immune system to fight pathogens. Different classes of antibodies have distinct structures and functions.
Step 1: Understanding IgG.
IgG is the most abundant immunoglobulin in human serum and plays a crucial role in secondary immune response.
Step 2: Placental transfer.
IgG is the only class of antibody that can cross the placenta from mother to fetus.
Step 3: Significance.

• Provides passive immunity to the fetus
• Protects newborns against infections during early life

Step 4: Eliminating other options.

• IgA: Found in secretions like saliva and milk
• IgM: First antibody produced but too large to cross placenta
• IgE: Involved in allergic reactions

Step 5: Conclusion.
Thus, IgG is the only antibody capable of crossing the human placenta.

Concept: Major Histocompatibility Complex (MHC) molecules are cell surface proteins that play a crucial role in immune response by presenting antigenic peptides to T-cells.
Step 1: Understanding MHC Class I.
MHC Class I molecules are present on the surface of all nucleated cells in the body.
Step 2: Function.
They present endogenous (intracellular) antigens to cytotoxic T-cells (CD8+ T cells).
Step 3: Comparison with other classes.

• MHC Class II: Found only on antigen-presenting cells (macrophages, dendritic cells, B-cells)
• MHC Class III: Involved in complement proteins, not antigen presentation

Step 4: Importance.
This widespread presence allows the immune system to monitor all cells for infection or abnormal changes.
Step 5: Conclusion.
Thus, MHC Class I molecules are found on all nucleated cells.