The Immune Response

In short: The immune system defends the body against pathogens in three lines. The first line is barriers like skin and mucus, the second is non-specific responses like phagocytes, inflammation and fever, and the third is the specific response of B cells, T cells and antibodies, which leaves memory cells for immunity.

Starting point: what you already know

• Pathogens include viruses, bacteria, fungi and protists (see the previous chapter).
• Blood contains white blood cells (see Transport in Animals).
• Cells have proteins on their membranes that act like ID tags (see The Cell Membrane and Transport).

Antigens: how the body tells self from non-self

An antigen is a molecule (usually a protein) that the immune system recognises. Your own cells carry "self" markers called MHC proteins. Pathogens carry foreign antigens, so the body can tell they are not you.

Three lines of defence

First line.

• Skin: a tough, dry barrier. Sweat and oils contain chemicals that kill microbes.
• Mucous membranes and mucus trap pathogens. Cilia sweep mucus out of the airways.
• Chemicals: stomach acid, and lysozyme (an enzyme) in tears and saliva.
• Reflexes: coughing, sneezing, vomiting.
• Good bacteria on the skin and in the gut outcompete pathogens.

Second line.

• Phagocytes (neutrophils and macrophages) engulf and digest pathogens by phagocytosis.
• Inflammation: damaged tissue releases histamine. Blood vessels widen and leak, so the area becomes red, warm, swollen and sore, and more white blood cells arrive.
• Fever: a higher body temperature slows many pathogens and speeds up immune cells.
• Natural killer cells destroy body cells infected with viruses.
• Complement proteins in the blood punch holes in bacteria.

Third line (adaptive immunity). Done by lymphocytes, white blood cells made in the bone marrow:

Antibodies

An antibody is a Y-shaped protein made by B cells (as plasma cells). The tips of the Y fit one specific antigen, like a lock and key.

Antibodies

• clump pathogens together (agglutination) so phagocytes can eat them,
• cover viruses so they cannot enter cells (neutralisation), and
• mark pathogens for destruction.

Immune memory

After an infection, some B and T cells become long-lived memory cells.

• First exposure (primary response): slow, about 1 to 2 weeks, small antibody response, so you get sick.
• Second exposure (secondary response): fast, strong and long. Usually no symptoms. You are immune.

Plant defences

Plants have no immune cells, but they still fight back.

• Physical barriers: waxy cuticle, bark, thick cell walls, thorns and hairs.
• Chemical defences: toxins, oils (like eucalyptus oil) and antimicrobial chemicals.
• Responses to attack: plants seal off infected areas, kill their own infected cells to stop the spread, close stomata, and send chemical alarm signals to the rest of the plant.

Quick summary

• Antigens are molecules the immune system recognises as self or non-self.
• First line: barriers like skin, mucus and acid. Second line: phagocytes, inflammation and fever. Both are innate and non-specific.
• Third line: B cells make specific antibodies. T cells coordinate and kill infected cells.
• Memory cells give a faster, stronger secondary response: immunity.
• Plants defend themselves with barriers, chemicals and by killing infected cells.

Quiz