Question
Differentiate between innate and adaptive immunity. Explain the difference between humoral and cell-mediated immune responses.
(NEET 2023, similar pattern)
Solution — Step by Step
Innate immunity is the body’s first line of defence — present from birth, does not require prior exposure to a pathogen, and responds the same way to all invaders. It includes:
- Physical barriers: Skin, mucous membranes, cilia in respiratory tract
- Chemical barriers: Lysozyme in tears/saliva, HCl in stomach, sebum on skin
- Cellular defences: Neutrophils, macrophages, natural killer (NK) cells — these perform phagocytosis and kill infected cells non-specifically
- Inflammatory response: Swelling, redness, heat, pain — brings more immune cells to the infection site
- Complement system: Proteins in blood that help destroy pathogens by lysis or opsonisation
Adaptive immunity develops after exposure to a specific pathogen. It is slower to activate (takes days) but is highly specific and produces immunological memory — making the second response faster and stronger.
Key features: specificity, diversity, memory, and self/non-self discrimination.
Adaptive immunity is carried out by lymphocytes — B cells and T cells — and has two arms.
Mediated by B lymphocytes and their antibodies:
- B cells recognise antigens and, with help from T-helper cells, differentiate into plasma cells (which secrete antibodies) and memory B cells.
- Antibodies (immunoglobulins) circulate in the blood and body fluids (humors), hence the name.
- Antibodies neutralise toxins, opsonise pathogens (mark them for phagocytosis), and activate complement.
- Effective against extracellular pathogens — bacteria in the blood, toxins, free virus particles.
Mediated by T lymphocytes:
- Cytotoxic T cells (killer T cells / CD8+): Directly kill infected cells by recognising viral antigens on their surface and releasing perforins and granzymes.
- Helper T cells (CD4+): Do not kill directly but activate both B cells and cytotoxic T cells by releasing cytokines.
- Effective against intracellular pathogens — viruses hiding inside cells, intracellular bacteria, cancer cells, transplanted tissues.
This is why HIV is so devastating — it destroys helper T cells (CD4+), crippling both arms of adaptive immunity.
Why This Works
The two-tier system (innate + adaptive) provides both immediate defence and long-term protection. Innate immunity holds the fort while adaptive immunity prepares a targeted response. The memory component of adaptive immunity is why vaccines work — a weakened or killed pathogen primes the immune system, so the real infection triggers a rapid, powerful secondary response.
| Feature | Innate | Adaptive |
|---|---|---|
| Specificity | Non-specific | Highly specific |
| Response time | Immediate (0-12 hours) | Slow (days to weeks) |
| Memory | No | Yes |
| Present at birth | Yes | Develops after exposure |
NEET frequently tests: “Which type of immunity is responsible for graft rejection?” Answer: cell-mediated immunity (T cells attack foreign tissue). Also commonly asked: “Which cells are affected by HIV?” Answer: Helper T cells (CD4+).
Common Mistake
Students often write that B cells are involved in cell-mediated immunity and T cells in humoral immunity — getting the two mixed up. The clear distinction: B cells = humoral (antibodies in body fluids), T cells = cell-mediated (direct cell killing). Use the mnemonic: B for Blood (humoral = in body fluids), T for Touch (cell-mediated = direct contact).
Another error: stating that innate immunity has no cells involved. Innate immunity includes several cell types — macrophages, neutrophils, dendritic cells, and NK cells. The difference is that these cells respond non-specifically, while adaptive immune cells (B and T lymphocytes) respond to specific antigens.