Malaria is caused by protozoa of the genus Plasmodium ('spp.' means several species). Five species infect humans: P. falciparum, P. vivax, P. ovale, P. malariae and P. knowlesi. P. falciparum is the most dangerous and causes most malaria deaths, mainly in sub-Saharan Africa. P. vivax is widespread in Asia and South America.Plasmodium is a parasite with two hosts:
• the human, where it reproduces asexually
• the female Anopheles mosquito, the vector, where sexual reproduction takes place.
• sporozoites: injected by the mosquito, they infect liver cells
• merozoites: produced by asexual division in liver cells and then in red blood cells
• gametocytes: sexual forms in the blood, taken up by a feeding mosquito, in which they form gametes that fuse and eventually produce new sporozoites.Each form has different surface antigens, so antibodies made against one stage may not recognise the next. By spending most of its time in the human inside liver cells and red blood cells, the parasite is hidden from much of the immune system. Red blood cells have no nucleus and do not display antigens on MHC proteins, so infected red blood cells are not destroyed by T killer cells.P. vivax and P. ovale can also form dormant liver stages (hypnozoites) that reactivate months or years later, causing relapses.
• the human, where it reproduces asexually
• the female Anopheles mosquito, the vector, where sexual reproduction takes place.
• sporozoites: injected by the mosquito, they infect liver cells
• merozoites: produced by asexual division in liver cells and then in red blood cells
• gametocytes: sexual forms in the blood, taken up by a feeding mosquito, in which they form gametes that fuse and eventually produce new sporozoites.Each form has different surface antigens, so antibodies made against one stage may not recognise the next. By spending most of its time in the human inside liver cells and red blood cells, the parasite is hidden from much of the immune system. Red blood cells have no nucleus and do not display antigens on MHC proteins, so infected red blood cells are not destroyed by T killer cells.P. vivax and P. ovale can also form dormant liver stages (hypnozoites) that reactivate months or years later, causing relapses.