krit.club logo

Biology - Diseases and Immunity

Grade 9IGCSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

•

A pathogen is a disease-causing organism, such as bacteria (Vibrio choleraeVibrio\ cholerae), viruses (HIVHIV), fungi, and protozoa.

•

Transmissible diseases are those in which the pathogen can be passed from one host to another via direct contact (blood, body fluids) or indirect contact (contaminated surfaces, food, water, air).

•

Body defenses include mechanical barriers (skin, nasal hairs) and chemical barriers (stomach acid consisting of HClHCl, mucus produced by goblet cells).

•

Phagocytosis is the process where phagocytes engulf and digest pathogens using enzymes.

•

Lymphocytes produce specific antibodies that bind to antigens on the surface of pathogens, leading to their destruction or marking them for phagocytes.

•

The specific shape of an antibody molecule is complementary to the specific shape of the antigenantigen it targets.

•

Active immunity is the defense against a pathogen by antibody production in the body, acquired after an infection or a vaccination.

•

Passive immunity is a short-term defense against a pathogen by antibodies acquired from another individual, such as a mother to an infant through the placenta or breast milk.

•

Vaccination involves weakened or dead pathogens (or their antigens) being introduced to the body to stimulate the production of memory cellsmemory\ cells for long-term immunity.

•

Type 1 diabetes is an example of an autoimmune disease where the immune system attacks and destroys insulin-producing cells in the pancreas.

📐Formulae

Magnification=Image sizeActual sizeMagnification = \frac{\text{Image size}}{\text{Actual size}}

Actual size (A)=Image size (I)Magnification (M)\text{Actual size (A)} = \frac{\text{Image size (I)}}{\text{Magnification (M)}}

1 mm=1000 μm1\ mm = 1000\ \mu m

💡Examples

Problem 1:

A bacterial cell, Vibrio choleraeVibrio\ cholerae, is measured to be 2 μm2\ \mu m in length. If an image of this bacterium is produced using a microscope with a magnification of ×5000\times 5000, calculate the length of the image in mmmm.

Solution:

I=A×M=2 μm×5000=10,000 μm=10 mmI = A \times M = 2\ \mu m \times 5000 = 10,000\ \mu m = 10\ mm

Explanation:

To find the image size, multiply the actual size (2 μm2\ \mu m) by the magnification (50005000). Convert the final answer from micrometers to millimeters by dividing by 10001000.

Problem 2:

Explain why a person who has already had measles is unlikely to catch it again when exposed to the same virus (MorbillivirusMorbillivirus).

Solution:

During the first infection, lymphocyteslymphocytes produce specific antibodies and memory cellsmemory\ cells. Upon re-exposure, the memory cellsmemory\ cells recognize the antigensantigens and produce large quantities of antibodies rapidly.

Explanation:

This is the principle of active immunity. The secondary immune response is much faster and produces a higher concentration of antibodies, neutralizing the virus before symptoms develop.