Page 115 - Biology Form Two
P. 115
Biology for Secondary Schools
White blood cells
These are irregular shaped cells with
nuclei as shown in Figure 5.9. They
are also called leucocytes. They help Pathogen
the body to fight against diseases and Nucleus
FOR ONLINE READING ONLY
infections. One milliliter of blood has
approximately 5,000 to 10,000 white
blood cells. They are produced in the Figure 5.10: Phagocytosis in a white blood
white bone marrow and in the lymph cell
nodes. White blood cells do not possess (ii) producing substances that
haemoglobin. White blood cells called neutralise toxins produced by
macrophages are capable of motility. pathogens;
Their motility enables them to get into (iii) causing clumping together of
the site of infection. foreign materials in the body; and
(iv) killing infected body cells.
Nucleus The effects of HIV on white blood
cells
The Human Immunodeficiency Virus
(a) (b)
(HIV) attacks a type of white blood
cells called T-helper cells. These cells
are essential for body immunity. When
they encounter an antigen, the T-helper
Nucleus cells divide themselves to form new
(c) (d)
cells. This increases the number of
Figure 5.9: White blood cells of different cells available to fight the infection.
shapes After the infection, some cells remain
as memory cells to activate an immune
Functions of white blood cells response if the infection happens again.
The white blood cells protect the body In addition, the T-helper cells activate
against infection. They perform this other cells in the immune system.
function by: HIV has a protein envelope that
(i) engulfing and destroying pathogens, can only bind to its receptor called
a process called phagocytosis, as CD4. The CD4 are found on the cell
shown in Figure 5.10; membrane of the T-helper cell. When
it enters the human body, HIV fuses
its protein envelope with the CD4 and
Student’s Book - Form Two 109
05/10/2024 15:36:39
BIOLOGY FORM 2 NEW.indd 109 05/10/2024 15:36:39
BIOLOGY FORM 2 NEW.indd 109
