Immunology forms the foundation of modern medicine. From prevention through vaccination with vaccines, to allergies and autoimmune diseases caused by excessive or abnormal immune responses, and HIV/AIDS, which destroys the immune system, we will broadly study the medical significance of immunity.
Basic Knowledge
Vaccine (vaccination): A method of creating immune memory without causing disease by inoculating the body with weakened or inactivated pathogens or specific antigens. Memory cells (memory B cells and memory T cells) are formed during the primary response, making the secondary response faster and stronger when the body is infected by the actual pathogen. Recent mRNA vaccines (mRNA vaccines against the novel coronavirus) are a new type that inoculates the body with mRNA for an antigen protein.
Allergy (type I hypersensitivity): IgE is produced against harmless substances such as pollen, food, and mites called allergens. Sensitization is completed when IgE binds to Fcε receptors on mast cells and basophils. During re-exposure, when an allergen cross-links IgE, histamine and leukotrienes are released all at once → runny nose, hives, bronchospasm, and, in the most severe cases, anaphylactic shock.
Autoimmune diseases: Diseases in which immune tolerance breaks down and T cells and B cells attack the body's own tissues. Examples: systemic lupus erythematosus (SLE) = autoantibodies against nucleic acids. Rheumatoid arthritis (RA) = T-cell infiltration and inflammation of the synovial membrane of the joints. Type 1 diabetes = CTL attack on pancreatic β cells. Multiple sclerosis (MS) = attack on the myelin sheath.
HIV/AIDS: The human immunodeficiency virus (HIV) infects and ultimately destroys CD4⁺ helper T cells. When the number of CD4⁺ T cells falls below <200/μL, the condition is diagnosed as AIDS (acquired immunodeficiency syndrome). Both cell-mediated and humoral immunity become dysfunctional → opportunistic infections such as Pneumocystis pneumonia and Kaposi's sarcoma develop. Today, disease progression can be suppressed with antiretroviral therapy (ART).
Explain, from the viewpoint of memory cells, why vaccination makes the secondary response faster and stronger.
Answer: Antigen-specific memory B cells and memory T cells survive for a long period as a result of vaccination (the primary response). When these memory cells are exposed to the same antigen again (the secondary response), they are activated and proliferate more rapidly than naive B cells and T cells, producing large amounts of antibodies and cytotoxic T cells. As a result, pathogens can be eliminated quickly during the early stages of infection.
- Vaccine = memory cells form during the primary response → the secondary response is faster
- IgE + mast cells = the main mechanism of allergies
- Anaphylaxis = severe type I allergy and a life-threatening condition
- Autoimmune disease = breakdown of immune tolerance → attack on the body's own tissues
- SLE = autoantibodies against nucleic acids and damage to multiple organs
- HIV = destroys CD4⁺ T cells → AIDS
- ART = antiretroviral therapy that suppresses HIV replication
Points to Note
① Vaccines do not “treat” infectious diseases; they “prevent” them. ② Autoimmune diseases and allergies are both “excessive immune responses,” but they differ in whether the target is the body's own tissues (autoimmunity) or harmless foreign substances (allergy). ③ HIV is an RNA virus with reverse transcriptase; it makes DNA by reverse transcription and integrates it into the host genome (a retrovirus).
Practice
- Explain the roles of IgE and mast cells in allergies in two stages: sensitization and re-exposure.
- Give two examples of autoimmune diseases and state which body's own tissues are attacked in each.
- Explain the mechanism by which HIV causes AIDS, including its relationship with CD4⁺ T cells.