'We did it': Tears and relief as poorly Zac, 4, starts his treatment in America

Zac has had the first stage of his CAR-T Therapy at the Children’s Hospital of Philadelphia.

Four-year-old Zac has an incredibly rare form of childhood leukaemia called Near Haploid. He is believed to be the only child in the UK to have the condition.

The Shropshire community came together to support Zac and his family, so far raising more than £630,000 through sponsored events, donations and a dress red for Zac day. The fundraising is continuing to help cover further costs over the coming months.

How you can continue to help:

• Anyone who wants to support the fundraising campaign should visit justgiving.com/crowdfunding/zac-oliver

• People can also join Zac's Facebook support page which is called 'Zachariah's's fight against Acute Lymphoblastic Leukaemia'.

• People can also donate using their mobile phone by texting ZACH75 £1 to 70070.

On Wednesday Zac began his CAR-T Therapy, which it is estimated will give him a 60 to 80 per cent chance of survival.

T-Cells were harvested from his blood, which will be modified in a laboratory over the next six weeks. In January they will be injected back into his body.

But mum Hannah Oliver Willets said the first day was ‘the most horrendous of my life’.

She took to Twitter to share the story with Zac's 1,600 followers.

"The last few days have been really tough. Zac was taken completely out of his comfort zone," she said.

"He didn't know any doctors or nurses, he didn't know how to navigate his way around the hospital and he missed his family and friends.

"When we arrived at the hospital , Zac started off by crying, running away, screaming and begging me to take him to his normal hospital.

"The staff were really good with him. They immediately called for a play specialist who came and helped, which worked wonders.

"We had a full day of back-to-back meetings with the various teams who will be involved in Zac's care. The CAR-T doctor said that she would be happy for Zac to return home for his bridging chemo because his organ function and general health is so good at present."

Hannah added: "This is the direct result of everyone's hard work and dedication to have raised the money in time. Zac is extremely happy about this as he can't wait to get home."

She explained how Zac became so distressed he needed a sedative: "The next day Zac's T-cells were harvested. Zac screamed the place down and wouldn't let any of the nurses or doctors near him.

"He was given a sedative to try and calm him down but he had an adverse reaction to this and became even more anxious, kicking, screaming and trying to run off.

"A second sedative was administered whilst three members of staff restrained him. This didn't work either. So they decided to get the big guns out and give him a general anesthetic. A large cannula was inserted into Zac's neck into the large vein.

"Once Zac came round he was transferred to the apheresis department where he was hooked up to a machine for around four to five hours. Zac's entire volume of blood was taken out of his body a total of four times... crazy, hey?

"The next day Zac had a reaction to the cleaning agent which they used to sterilise his skin. Zac has a very sore neck and chest.

"After all this Zac is definitely ready to come back to the UK where he can be treated for the next six weeks under the care of his normal hospital."

Hannah finished: "So although it's been really hard, we did it. We have a bag full of T-cells which are being sent to be genetically modified and prepared for his re-infusion around Christmas time."