Barriers, innate defence and adaptive immunity

What you’ll learn

• Explain barriers and non-specific immune responses.
• Link antigen presentation to lymphocyte activation.
• Distinguish the roles of B cells and T cells.

Understand the idea

Pathogens can enter through the respiratory or digestive tract, damaged skin and other exposed surfaces. Intact skin forms a physical barrier; stomach acid damages many swallowed organisms; resident microbiota compete for space and nutrients. Lysozyme hydrolyses bacterial cell-wall components. Inflammation increases local blood flow and vascular permeability, helping defensive cells and proteins reach affected tissue. Interferons released during viral infection induce antiviral responses in nearby cells.

Phagocytes engulf material into a vesicle, which fuses with lysosomes whose enzymes digest it. Macrophages can also display pathogen antigens on their surface as antigen-presenting cells. An antigen is a molecular feature recognised by a specific immune receptor; antibodies have complementary binding sites. Antibody binding may neutralise a pathogen or toxin, clump particles and aid recognition by phagocytes.

A matching lymphocyte is selected and undergoes clonal expansion. Activated B cells form antibody-secreting plasma cells and memory B cells. Helper T cells release signals that coordinate other immune cells; cytotoxic T cells destroy infected host cells displaying appropriate antigens. Memory T cells also persist. A secondary response can therefore be faster and larger than the first encounter, reducing the chance of symptoms.

Worked example

Explain the sequence linking uptake of a pathogen by a macrophage to production of specific antibodies.

1. The macrophage digests the pathogen and presents its antigens.
2. A helper T cell with a matching receptor is activated and provides signals that help activate an appropriate B cell.
3. The selected B cell divides; plasma-cell descendants secrete antibodies with binding sites complementary to the antigen.

Answer: Antigen presentation and helper signalling connect innate recognition to the specific, amplified antibody response.