Explain how the immune system defends the body against pathogens using white blood cells and antibodies, how immunity and vaccination work, and how disease disrupts homeostasis (NYSSLS LS1, cause and effect; stability and change).

What this topic is asking

NYSSLS LS1 wants you to explain how the body defends itself against pathogens and how vaccines work, and to connect disease to the loss of homeostasis. On the Life Science: Biology Regents this very often comes as a cluster with an antibody-level graph (a primary versus secondary response). The crosscutting concepts are cause and effect and stability and change.

Pathogens and disease

When a pathogen enters the body, it can damage cells or release toxins, causing the symptoms of disease. The body's job is to detect and destroy the pathogen before it does serious harm.

How the immune system defends the body

The immune system uses white blood cells to defend the body in two main ways:

• Some white blood cells engulf and digest pathogens directly.
• Others (lymphocytes) produce antibodies, proteins that bind to the pathogen's antigens, marking it for destruction or neutralizing it.

Immunity and the secondary response

After an infection, some lymphocytes remain as memory cells. If the same pathogen is met again, these memory cells recognize it and produce antibodies much faster and in greater amounts than the first time. This secondary response is usually quick enough to destroy the pathogen before it causes illness, which is what we mean by being immune. The classic exam graph shows a small, slow primary response and a large, fast secondary response, and asks you to explain the difference using memory cells.

Vaccination

A vaccine contains a harmless form or part of a pathogen (for example a weakened or dead microbe, or one of its antigens). It triggers the immune system to make antibodies and, crucially, memory cells, without causing the disease. If the real pathogen is later encountered, the body mounts a fast, large secondary response and the person does not become ill. Widespread vaccination also protects communities by reducing the spread of the pathogen.

Disease and homeostasis

A disease can disrupt homeostasis by damaging cells, destroying tissue, or interfering with a feedback system, so that the body can no longer keep a variable within its normal range (see homeostasis and feedback). For example, a disease that destroys the cells that make insulin disrupts the control of blood glucose. This links the immune topic back to the central idea of a stable internal environment.

Try this

Q1. Explain what is meant by an antibody being specific. [2]

• Cue. Its shape matches and binds only one particular antigen, so it acts against only that pathogen.

Q2. Explain how a vaccine protects a person from a disease. [2]

• Cue. It introduces a harmless form or part of the pathogen, so the body makes memory cells without illness; a later infection triggers a fast, large secondary response that prevents the disease.