Humoral immunity · ImmuneLab

The body's three lines of defence

To cause disease, a pathogen must first break through three lines of defence. The first is the skin and mucous membranes; the second is the bactericidal substances in body fluids (such as lysozyme) and the phagocytes. Everyone is born with these two, and they do not target any one pathogen, so they are called non-specific immunity.

The third line is made up of immune organs and immune cells (mainly lymphocytes). It targets one particular pathogen only, so it is called specific immunity, and it takes a few days to work.

Choose a route of invasion on the right, then turn off a line of defence and see what the pathogen does.

How humoral immunity works

After a pathogen enters the body fluids, a B cell needs two signals to be activated: first, the pathogen contacts the B cell directly; second, particular molecules on the surface of a helper T cell change and bind to the B cell. The helper T cell first receives the antigen from an antigen-presenting cell (dendritic cell, B cell or macrophage).

Helped by cytokines, the B cell divides and differentiates: most become plasma cells, which make and secrete antibodies; a few become memory B cells. Antibodies bind to the pathogen, stopping it from multiplying and from attaching to body cells; the complexes then form a precipitate that other immune cells engulf and digest.

Use the buttons below to go step by step, or press "Autoplay". Remove helper T cells or cytokines on the right to see what happens without them.

Secondary response and vaccines

Memory B cells can survive for years or even decades. When the same antigen enters again, memory cells rapidly proliferate and differentiate, producing large numbers of plasma cells and antibodies. The secondary response is faster and stronger than the primary one, and the pathogen is often wiped out before it can multiply.

A vaccine is usually an inactivated or weakened pathogen (or part of one). It does not cause disease, yet it makes the body produce antibodies and memory cells. Some vaccines need two or three doses, using the secondary response to boost the effect.

The timeline moves on by itself: vaccinate first, wait a while, then infect with the pathogen and compare the curves; then try a variant.

Immune disorders: allergy

An immune system that is too strong or too weak can both cause disease. An allergic reaction is tissue damage or disordered function in an already immunised body when it meets the same antigen again: antibodies made the first time the allergen entered attach to mast cells; when it enters again it binds to them, and the mast cells release histamine, which widens the capillaries and makes them more permeable.

In autoimmune diseases the immune system attacks the body's own components as if they were foreign, as in rheumatic heart disease and systemic lupus erythematosus. Immunodeficiency diseases are caused by too little or no immune function, as in AIDS, where HIV mainly attacks helper T cells.

In-class testing

Look back over the four topics and answer the multiple-choice questions below.