Question
Differentiate between innate and adaptive immunity. Explain the difference between humoral and cell-mediated immune responses.
Solution — Step by Step
flowchart TD
A[Immune System] --> B[Innate Immunity - Non-specific]
A --> C[Adaptive Immunity - Specific]
B --> B1[Physical barriers - Skin, Mucus]
B --> B2[Cellular - Phagocytes, NK cells]
B --> B3[Chemical - Lysozyme, HCl, Interferons]
C --> D[Humoral - Antibody mediated]
C --> E[Cell-mediated - T cell mediated]
D --> D1[B lymphocytes produce antibodies]
D --> D2[Targets: extracellular pathogens]
E --> E1[T lymphocytes destroy infected cells]
E --> E2[Targets: intracellular pathogens, transplants]Innate immunity is non-specific — it does not distinguish between different pathogens. It is present from birth. It includes: (a) Physical barriers: skin, mucous membranes, cilia, (b) Chemical barriers: lysozyme in tears/saliva, HCl in stomach, sebum on skin, (c) Cellular barriers: phagocytes (neutrophils, macrophages) engulf pathogens, NK (natural killer) cells destroy virus-infected cells, (d) Inflammatory response and fever.
Adaptive immunity is specific to a particular pathogen. It develops after exposure (takes 4-7 days on first encounter). It has memory — the second response is faster and stronger. It involves two types of lymphocytes: B cells and T cells, both originating from bone marrow stem cells.
B lymphocytes produce antibodies (immunoglobulins) that circulate in blood and lymph. Antibodies bind to antigens on the surface of extracellular pathogens (bacteria, toxins, viruses before they enter cells), neutralising them or marking them for destruction by phagocytes. There are 5 classes: IgG, IgA, IgM, IgD, IgE.
T lymphocytes handle intracellular pathogens (viruses inside cells, intracellular bacteria, cancer cells, transplanted tissue). Types: Cytotoxic T cells (Tc/CD8+) directly kill infected cells. Helper T cells (Th/CD4+) activate both B cells and cytotoxic T cells by releasing cytokines. CMI is also responsible for transplant rejection.
Why This Works
The two arms of adaptive immunity complement each other. Humoral immunity handles threats in body fluids (extracellular). Cell-mediated immunity handles threats hiding inside cells (intracellular). Together with innate immunity, they provide comprehensive protection. Memory cells (both B and T) ensure a rapid secondary response upon re-exposure — this is the basis of vaccination.
Alternative Method
| Feature | Innate Immunity | Adaptive Immunity |
|---|---|---|
| Specificity | Non-specific | Highly specific |
| Memory | No memory | Has memory |
| Response time | Immediate (0-4 hours) | Slow first time (4-7 days), fast on re-exposure |
| Components | Skin, phagocytes, NK cells | B cells, T cells, antibodies |
| Present from | Birth | Develops after exposure |
Common Mistake
Students confuse active and passive immunity with humoral and cell-mediated. Active immunity is when the body produces its own antibodies (after infection or vaccination). Passive immunity is when ready-made antibodies are transferred (mother to fetus via placenta, or injection of antiserum). Both active and passive can be humoral — they are different classifications.