The immune system

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

Test this topic free All Biology AS topics

The idea

An antigen is a molecule, often a protein or glycoprotein on a cell surface, that triggers an immune response. Self antigens belong to your own cells and cause no response. Non-self antigens are foreign and are attacked.

Phagocytes (neutrophils and macrophages) engulf and digest pathogens. Lymphocytes are specific: each B-lymphocyte and T-lymphocyte has receptors that fit only one antigen.

In a primary response the few lymphocytes that fit the antigen are selected and divide by mitosis into a large clone. This takes days, so the person falls ill. Memory cells made at this time stay in the body. On a second infection they respond faster and make more antibody.

Rules to remember

Phagocytosis: attraction to the pathogen, attachment, engulfing into a phagocytic vacuole, fusion with lysosomes, digestion by enzymes.
Neutrophils: short-lived, travel in the blood. Macrophages: long-lived, stay in tissues and present antigens to lymphocytes.
T-helper cells release cytokines that stimulate B-lymphocytes to divide and macrophages to carry out more phagocytosis.
T-killer cells attach to infected body cells and release toxins that kill them.
Activated B-lymphocytes divide to form plasma cells, which secrete antibodies, and memory cells.
Secondary response: shorter delay, faster rise and much higher antibody concentration than the primary response.

Common mistake

Saying that memory cells or T cells secrete antibodies. Only plasma cells secrete antibodies; memory cells must first divide to form plasma cells.

Worked example

A person has normal numbers of B-lymphocytes but very few T-helper cells. Why does this person make little antibody against a new pathogen?

B-lymphocytes with the matching receptor bind the antigen, but they need a signal to become fully active.
That signal is the cytokines released by T-helper cells specific to the same antigen.
With few T-helper cells, few B-lymphocytes divide to form plasma cells.
Few plasma cells means little antibody is secreted.

Answer: B-lymphocytes are not stimulated by T-helper cytokines, so few plasma cells form.

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Read the full notes

Full-course notes

Keep going

← Antibiotics Antibodies and vaccination →Take a Biology AS test