Innate vs Adaptive Immunity

The human immune system has two main arms. The innate immune system provides rapid, non-specific defence present from birth. The adaptive immune system is slower to activate but delivers a highly specific response, complete with immunological memory.

Lines of Defence

First Line (Innate)

Physical & chemical barriers: skin, mucous membranes, stomach acid, tears

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Second Line (Innate)

Phagocytes, inflammation, fever, complement proteins, NK cells

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Third Line (Adaptive)

B lymphocytes (antibodies) and T lymphocytes (cell-mediated immunity)

Key point: Innate immunity is fast but non-specific -- it responds the same way to all pathogens. Adaptive immunity takes days to develop on first exposure but is antigen-specific and produces memory cells for faster future responses.

Antibodies and Lymphocytes

Antibodies (immunoglobulins) are Y-shaped proteins produced by plasma cells (differentiated B lymphocytes). Each antibody has a unique antigen-binding site that recognises a specific epitope on a pathogen's surface antigen. T lymphocytes coordinate and carry out cell-mediated immune responses.

Key Immune Cells

B Lymphocytes

• Mature in bone marrow
• Produce antibodies (humoral immunity)
• Differentiate into plasma cells & memory B cells
• Each B cell has unique surface antibody

T Lymphocytes

• Mature in thymus
• Helper T cells (CD4+) activate B cells
• Cytotoxic T cells (CD8+) destroy infected cells
• Memory T cells provide long-term protection

Antibody Functions

Neutralisation: Antibodies bind to pathogens and toxins, blocking their ability to infect cells.

Agglutination: Antibodies clump pathogens together, making them easier for phagocytes to engulf.

Opsonisation: Antibodies coat pathogens, tagging them for destruction by phagocytes.

Vaccination and Immunological Memory

Vaccination introduces a weakened, inactivated, or fragment of a pathogen (antigen) to stimulate the adaptive immune system without causing disease. This produces memory B and T cells, so a subsequent exposure triggers a faster, stronger secondary immune response.

Primary vs Secondary Immune Response

Primary Response

Slow onset (7-14 days), lower antibody levels, mainly IgM produced

Secondary Response

Rapid onset (1-3 days), much higher antibody levels, mainly IgG produced

Types of Immunity

1. Active natural: Immunity gained from infection.
2. Active artificial: Immunity gained from vaccination.
3. Passive natural: Antibodies transferred from mother (placenta/breast milk).
4. Passive artificial: Injection of pre-formed antibodies (e.g., anti-venom).

Key Vocabulary

Antigen

Any molecule (usually a protein) on the surface of a pathogen that the immune system recognises as foreign and can trigger an immune response.

Antibody

A Y-shaped immunoglobulin protein produced by plasma cells that specifically binds to an antigen, marking it for destruction or neutralising it.

Phagocytosis

The process by which phagocytes (e.g., macrophages, neutrophils) engulf and digest pathogens and cellular debris as part of innate immunity.

Immunological Memory

The ability of the adaptive immune system to remember previously encountered antigens via long-lived memory B and T cells, enabling a faster secondary response.

Worked Examples

1

Explain why a person who has recovered from chickenpox rarely gets it again.

Step 1: During the primary infection, the adaptive immune system produced B and T cells specific to the varicella-zoster virus antigens.

Step 2: After recovery, long-lived memory B cells and memory T cells persist in the body.

Answer: On re-exposure, these memory cells mount a rapid, high-magnitude secondary immune response that eliminates the virus before symptoms develop, providing long-lasting immunity.

2

Distinguish between the roles of helper T cells and cytotoxic T cells.

Helper T cells (CD4+): Recognise antigens presented by antigen-presenting cells (APCs) on MHC II molecules. They release cytokines that activate B cells, cytotoxic T cells, and macrophages.

Cytotoxic T cells (CD8+): Recognise antigens on MHC I molecules of infected or abnormal cells. They release perforin and granzymes to induce apoptosis (programmed cell death) in the target cell.

3

Explain why vaccines may contain an adjuvant.

Step 1: Some vaccine antigens (especially inactivated or subunit vaccines) are weakly immunogenic on their own.

Step 2: An adjuvant is a substance added to enhance the immune response to the antigen.

Answer: Adjuvants (e.g., aluminium salts) slow the release of antigen at the injection site and stimulate innate immune cells, leading to a stronger and more sustained adaptive immune response and better immunological memory.

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

Which of the following is part of the innate immune system?

Antibody production by plasma cells Memory T cell activation Clonal selection of B cells Phagocytosis by macrophages

Question 2

The secondary immune response is faster and stronger than the primary response because:

Memory cells from the first exposure are rapidly reactivated More phagocytes are produced The pathogen is weaker the second time The skin barrier is stronger after first infection

Question 3

Which cell type is primarily responsible for producing antibodies?

Helper T cells Plasma cells (differentiated B cells) Cytotoxic T cells Natural killer cells

Question 4

Passive immunity differs from active immunity because passive immunity:

Is long-lasting and involves memory cells Requires the individual to be exposed to the pathogen Involves receiving pre-formed antibodies and does not produce memory Only involves T cell responses

Question 5

Cytotoxic T cells destroy infected body cells by:

Producing antibodies that bind to the cell Engulfing the cell through phagocytosis Releasing histamine to cause inflammation Releasing perforin and granzymes to trigger apoptosis

Human Immune System