Innate and adaptive immunity

MDCAT Biology · Biology · Immunity. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Innate immunity is present from birth and acts in the same way against every microbe. The first line of defence keeps microbes out: skin, sebum, mucus and cilia, stomach acid, and lysozyme in tears and saliva. The second line acts inside the body: phagocytes (neutrophils and macrophages), inflammation, fever and interferon.

Adaptive immunity is the third line. It is specific to one antigen and it has memory. B lymphocytes give antibody-mediated (humoral) immunity. T lymphocytes give cell-mediated immunity.

An antibody is a Y-shaped protein that binds to one particular antigen and marks it for destruction.

Rules to remember

Phagocytosis: attachment to the microbe → engulfing into a vacuole (phagosome) → lysosomes fuse with it → enzymes digest the microbe
Inflammation: mast cells release histamine → arterioles dilate and capillaries become leaky → redness, heat, swelling and pain
Interferon is released by virus-infected cells and makes neighbouring cells resist viruses
B cells mature in the bone marrow; T cells are made in the bone marrow and mature in the thymus
An activated B cell divides to form plasma cells (which secrete antibodies) and memory cells
Helper T cells activate B cells and killer T cells; killer (cytotoxic) T cells destroy infected body cells
Antibody = 2 heavy chains + 2 light chains joined by disulphide bonds; the variable regions at the tips form 2 antigen-binding sites

Common mistake

Do not say that B cells or T cells engulf microbes. Phagocytes (neutrophils and macrophages) engulf; plasma cells secrete antibodies; killer T cells kill infected cells by contact.

Worked example

A bacterium gets past the skin through a cut. Put these events in order: plasma cells secrete antibodies; a macrophage engulfs the bacterium; helper T cells are activated; B cells divide.

A phagocyte acts first: the macrophage engulfs the bacterium and shows its antigens on its surface.
Helper T cells that recognise this antigen are activated.
The helper T cells stimulate the matching B cells, which divide.
Most of the new cells become plasma cells and secrete antibodies.

Answer: Macrophage engulfs → helper T cells activated → B cells divide → plasma cells secrete antibodies.

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