Immunity

Our first line of defense against foreign organisms is barrier tissues such as the skin (mechanical barrier) that stop the entry of organism into our bodies. If, however, these barrier layers are penetrated, the body contains cells that respond rapidly to the presence of the invader. Other mechanical barriers include:

            -Movement of cilia in respiratory track

            -Peristalsis in alimentary canal

            -Flushing action of tears and saliva

            -Acidic stomach to kill microorganisms ¹

If these barriers did not stop the invaders (pathogens) then the blood would play an important role in defending the body (second line of defense). E.g. phagocytes (WBC) would engulf microbe and kill them, or lymphocytes would produce antibodies that would attach to antigens (found on cell surface of pathogens) and destroy the invading pathogen. Lymphocytes normally produce specific antibodies, and also recall previous attacks of the body.

Immunity can be grouped into two categories
1. Natural Immunity

2. Artificial Immunity

Natural Immunity

a. Actively acquired immunity: Antibody-antigen reaction

-cause pathogen to clump together

-disintegrate pathogen, e.g. make it more porous by damaging cell membrane

-neutralize toxins produced by pathogens

Production of antibodies takes some time to occur, and pathogen would have produced symptoms of the disease before they are destroyed by the immune system. However, if a person is infected a second time by the same pathogen, then the antibodies production would be more rapid, since the lymphocytes would have remembered the previous pathogenic attack

b. Passively acquired Immunity: This is a short lived immunity. It occurs during breast feeding, where breast milk contains antibodies to protect the new born against infections. Antibodies may also enter the feotus before birth through the placenta.

Artificial Immunity

a. Actively acquired Immunity: This is mainly through vaccination. The vaccine contains weakened

Vaccine 

live pathogen, or dead pathogens, that cannot cause the disease. However, lymphocytes would still recognize them (i.e. the antigen on their surface), and produce antibodies. The next time a pathogen enters the body, the immune system (lymphocyte) would immediately produce antibodies, since it already "learned" the specific antigen from the vaccine. This type of immunity may last for a short time or a lifetime. E.g. Typhoid vaccines last three years, while tuberculosis vaccine lasts a lifetime².

b. Passively Acquired Immunity: This is also though vaccination. However the vaccine contains the specific antibody or antitoxin that the body cannot make. This is a short lived immunity, since the lymphocytes cannot learn to make them. E.g. Hepatitis vaccine last for three months².

http://preventdisease.com/images/immune-system.jpg

References

1. http://www.microbiologybook.org/Infectious%20Disease/Sexually%20Transmitted%20Diseases.htm

2. Anne Tindale, Biology: A concise revision course for CXC. Stanley Thornes (Publishers) Ltd. 

3. Image: https://www.studyblue.com/notes/note/n/ch-15-adaptive-specific-immunity-and-immunization-/deck/638301