Question
Differentiate between innate and adaptive immunity. Explain the two branches of adaptive immunity — humoral and cell-mediated. How do they work together to fight infections?
(NEET, CBSE Class 12 — Human Health and Disease)
Solution — Step by Step
Present from birth, non-specific (acts against all pathogens equally). Four barriers: (1) Physical — skin, mucous membranes; (2) Physiological — acid in stomach, lysozyme in tears; (3) Cellular — neutrophils, macrophages (phagocytosis), NK cells; (4) Cytokine — interferons. No memory. Immediate response.
Develops after exposure to a pathogen. Specific — each response targets a particular antigen. Involves lymphocytes (B cells and T cells). Has immunological memory — faster and stronger response on re-exposure. This is why vaccination works.
Mediated by B lymphocytes. When activated by an antigen, B cells differentiate into plasma cells (produce antibodies/immunoglobulins) and memory B cells. Antibodies circulate in blood and lymph (humors), neutralise toxins, opsonise bacteria, and activate complement. Effective against extracellular pathogens (bacteria, toxins).
Mediated by T lymphocytes. Helper T cells (CD4+) activate B cells and other T cells. Cytotoxic T cells (CD8+) directly kill infected cells (virus-infected, cancer cells, transplanted tissue). Effective against intracellular pathogens and abnormal cells.
graph TD
A["Immune Response"] --> B["Innate Immunity<br/>Non-specific, No memory"]
A --> C["Adaptive Immunity<br/>Specific, Has memory"]
B --> D["Physical barriers<br/>Skin, Mucous"]
B --> E["Cellular<br/>Macrophages, NK cells"]
C --> F["Humoral<br/>B cells → Antibodies"]
C --> G["Cell-mediated<br/>T cells → Direct killing"]
F --> H["Targets: Extracellular<br/>pathogens, toxins"]
G --> I["Targets: Intracellular<br/>pathogens, cancer cells"]Why This Works
Innate immunity acts as the first responder — fast but generic. If the pathogen breaches innate defences, adaptive immunity kicks in — slower initially (takes 4-7 days) but highly specific and develops memory for future encounters.
The two branches of adaptive immunity complement each other: humoral immunity handles threats in body fluids (circulating bacteria, free viruses, toxins), while cell-mediated immunity handles threats hidden inside cells (virus-infected cells, intracellular bacteria like Mycobacterium).
Vaccination works by exposing the body to weakened or dead pathogens, triggering adaptive immunity and creating memory cells — without causing the actual disease. On real infection, memory cells mount a rapid, powerful response.
Alternative Method — Active vs Passive Immunity
Active immunity: body makes its own antibodies (natural infection or vaccination). Lasts long.
Passive immunity: pre-formed antibodies are given (mother to fetus via placenta, or injection of antiserum). Immediate but short-lived.
For NEET: antibodies are produced by plasma cells (derived from B cells). T cells do NOT make antibodies. HIV destroys helper T cells (CD4+), which is why it cripples the entire immune system — without helper T cells, neither B cells nor cytotoxic T cells can function properly.
Common Mistake
Students confuse humoral and cell-mediated immunity. The quick distinction: humoral = antibodies in fluids (B cells), cell-mediated = direct cell killing (T cells). If the question mentions “antibodies” or “immunoglobulins,” it is humoral. If it mentions “killing infected cells” or “transplant rejection,” it is cell-mediated.