1. Active immunity is produced by injecting the microbes deliberately during immunisation.
2. Antibodies are produced by the B-cells of our body in response to the antigens;they neutralise the pathogenic agents during actual infection.
3. The vaccines also generate memory B-cells and T-cells,which recognise the same pathogen on subsequent exposure and destroy them by a massive production of antibodies.

Two special types of lymphocytes in humans are B-lymphocytes and T-lymphocytes. B-lymphocytes execute antibody-mediated response while T-lymphocytes show cell-mediated response.

Ganja is obtained from the leaves,flower tops and the resin of the plant Cannabis sativa.

They interact with the cannabinoid receptors present in the brain and affect the cardiovascular system of the body.

Cocaine has a potent stimulating action on the central nervous system.

It produces a sense of euphoria and increased energy and hence,sportspersons become addicted to it.

(a) Amoebiasis is caused by Entamoeba histolytica.
(b) Symptoms:
(i) Constipation 
(ii) Abdominal cramps and pain.
(iii) Stool with excess mucus and blood clots.
Transmission 
Drinking water and food contaminated with the faeces of infected persons.
Houseflies act as mechanical carriers and transmit the parasite from faeces to food and water.

1. Life cycle of Plasmodium in human body.
2. When an infected female Anopheles mosquito bites a person,the sporozoites are injected into the body.
3. They reach the liver cells through blood.
4. The parasite reproduces asexually in the liver cells and by the bursting of liver cells,new cells are released into the blood.
5. They enter the red blood cells (RBCs) and reproduce asexually;by bursting the red blood cells,the cells of the parasite release haemozoin,which causes the cycles of fever,chill and shivering.
6. The released parasite cells infect new red blood cells;some continue the asexual reproduction and cause the cycles of fever.
7. Others enter the sexual stage and form gametocytes in the RBCs,which are picked by the Anopheles mosquito, along with the blood meal.

Ascariasis is caused by Ascaris lumbricoides.The intestine becomes blocked and damaged.
Symptoms
Intestinal bleeding - Muscular pain
Anaemia - Fever
Blockage of intestinal passage.
Transmission 
The eggs of the parasite from the faeces of infected person contaminate soil,water and plants. Infection is mainly through contaminated water,fruits and vegetables.

(i) Avoiding undue pressure on adolescents.
(ii) Seeking professional and medical help.

It is true that once a person gets entangled in the deadly whirlpool of alcohol or drugs, it is very difficult to get rid of this habit because, in the absence of a continuous supply of alcohol/drugs, the person starts showing certain symptoms called withdrawal signs which are listed below:
(1) Anxiety  
(2) Shakiness
(3) Nausea  
(4)Sweating.

Stem cells are undifferentiated biological cells that can differentiate into specialized cells and can divide (through mitosis) to produce more stem cells. They are found in multicellular organisms.
Types: In mammals, there are two broad types of stem cells, Embryonic stem cells, which are isolated from the inner cell mass ofblastocysts, and Adult stem cells, which are found in various tissues.
Role: In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenishing adult tissues. In a developing embryo, stem cells can differentiate into all the specialized cells-ectoderm, endoderm and mesoderm (see induced pluripotent stem cells)-but also maintain the normal turnover of regenerative organs, such as blood, skin, or intestinal tissues.
Sources of stem cells: There are three known accessible sources of autologous adult stem cells in humans:
1. Bone marrow, which requires extraction by harvesting, that is, drilling into bone (typically the femur or iliac crest),
2. Adipose tissue (lipid cells), which requires extraction by liposuction, and
3. Blood, which requires extraction through apheresis, wherein blood is drawn from the donor (similar to a blood donation), and passed through a machine that extracts the stem cells and returns other portions of the blood to the donor.
Stem cells can also be taken from umbilical cord blood just after birth. Of all stem cell types, autologous harvesting involves the least. risk. By definition, autologous cells are obtained from one's own body,just as one may bank his or her own blood for elective surgical procedures.
Uses of stem cells:
1. Adult stem cells are frequently used in medical therapies, for example in bone marrow transplantation.
2. Stem cells can now be artificially grown and transformed (differentiated) into specialized cell types with characteristics consistent with cells of various tissues such as muscles or nerves.
3. Embryonic cell lines and autologous embryonic stem cells generated through somatic cell nuclear transfer of dedifferentiation have also been proposed as promising candidates for future therapies.

The life cycle of a malarial parasite is shown below: