Sporozoites.
After entering the bloodstream of host it finally reaches the liver cell.

The concept of immunological surveillance postulates that the primary function of the immune system is to "seek and destroy" malignant cells that arise by somatic mutation. The efficiency of the surveillance mechanism reduces either as a result of ageing or due to congenital or acquired immunodeficiencies, leads to increased incidence of cancer. Thus, if immunological surveillance is effective, cancer should not occur. The development of tumour represents a lapse in surveillance.

(a) First generation vaccine - MMR vaccine
(b) Second generation vaccine - Hepatitis- B vaccine
(c) Third generation vaccine - DNA vaccine

The chief application of antigen-antibody reactions are to determine blood groups for transfusion, to study serological ascertainment of exposure to infectious agents, to develop immunoassays for the quantification of various substances, to detect the presence or absence of protein in serum and to determine the characteristics of certain immunodeficiency diseases

Viral diseases are generally classified into four types on the basis of the symptoms produced in the body organs.
(i) Pneumotropic diseases (Eg: respiratory tract infected by influenza)
(ii) Dermotropic diseases (Eg.skin and subcutaneous tissues affected by chicken pox and measles)
(iii) Viscerotropic diseases (Eg: blood and visceral organs affected by yellow fever and dengue fever)
(iv) Neurotropic diseases (Eg: central nervous system affected by rabies and polio)